Doc/gitsubmodules: make some changes to improve readability and syntax

* Only mention porcelain commands in examples

* Split a sentence for better readability

* Add missing apostrophes

* Clearly specify the advantages of using submodules

* Avoid abbreviations

* Use "Git" consistently

* Improve readability of certain lines

* Clarify when a submodule is considered active

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitsubmodules.txt

@@ -36,8 +36,8 @@ The `gitlink` entry contains the object name of the commit that the
 superproject expects the submodule’s working directory to be at.
 
 The section `submodule.foo.*` in the `.gitmodules` file gives additional
-hints to Gits porcelain layer such as where to obtain the submodule via
-the `submodule.foo.url` setting.
+hints to Git's porcelain layer. For example, the `submodule.foo.url`
+setting specifies where to obtain the submodule.
 
 Submodules can be used for at least two different use cases:
 
@@ -51,18 +51,21 @@ Submodules can be used for at least two different use cases:
 
 2. Splitting a (logically single) project into multiple
    repositories and tying them back together. This can be used to
-   overcome current limitations of Gits implementation to have
+   overcome current limitations of Git's implementation to have
    finer grained access:
 
-    * Size of the git repository:
+    * Size of the Git repository:
       In its current form Git scales up poorly for large repositories containing
       content that is not compressed by delta computation between trees.
-      However you can also use submodules to e.g. hold large binary assets
-      and these repositories are then shallowly cloned such that you do not
+      For example, you can use submodules to hold large binary assets
+      and these repositories can be shallowly cloned such that you do not
       have a large history locally.
     * Transfer size:
       In its current form Git requires the whole working tree present. It
       does not allow partial trees to be transferred in fetch or clone.
+      If the project you work on consists of multiple repositories tied
+      together as submodules in a superproject, you can avoid fetching the
+      working trees of the repositories you are not interested in.
     * Access control:
       By restricting user access to submodules, this can be used to implement
       read/write policies for different users.
@@ -73,9 +76,10 @@ The configuration of submodules
 Submodule operations can be configured using the following mechanisms
 (from highest to lowest precedence):
 
- * The command line for those commands that support taking submodule specs.
-   Most commands have a boolean flag '--recurse-submodules' whether to
-   recurse into submodules. Examples are `ls-files` or `checkout`.
+ * The command line for those commands that support taking submodules
+   as part of their pathspecs. Most commands have a boolean flag
+   `--recurse-submodules` which specify whether to recurse into submodules.
+   Examples are `grep` and `checkout`.
    Some commands take enums, such as `fetch` and `push`, where you can
    specify how submodules are affected.
 
@@ -87,8 +91,8 @@ Submodule operations can be configured using the following mechanisms
 For example an effect from the submodule's `.gitignore` file
 would be observed when you run `git status --ignore-submodules=none` in
 the superproject. This collects information from the submodule's working
-directory by running `status` in the submodule, which does pay attention
-to its `.gitignore` file.
+directory by running `status` in the submodule while paying attention
+to the `.gitignore` file of the submodule.
 +
 The submodule's `$GIT_DIR/config` file would come into play when running
 `git push --recurse-submodules=check` in the superproject, as this would
@@ -97,20 +101,20 @@ remotes are configured in the submodule as usual in the `$GIT_DIR/config`
 file.
 
  * The configuration file `$GIT_DIR/config` in the superproject.
-   Typical configuration at this place is controlling if a submodule
-   is recursed into at all via the `active` flag for example.
+   Git only recurses into active submodules (see "ACTIVE SUBMODULES"
+   section below).
 +
 If the submodule is not yet initialized, then the configuration
-inside the submodule does not exist yet, so configuration where to
+inside the submodule does not exist yet, so where to
 obtain the submodule from is configured here for example.
 
- * the `.gitmodules` file inside the superproject. Additionally to the
-   required mapping between submodule's name and path, a project usually
+ * The `.gitmodules` file inside the superproject. A project usually
    uses this file to suggest defaults for the upstream collection
-   of repositories.
+   of repositories for the mapping that is required between a
+   submodule's name and its path.
 +
-This file mainly serves as the mapping between name and path in
-the superproject, such that the submodule's git directory can be
+This file mainly serves as the mapping between the name and path of submodules
+in the superproject, such that the submodule's Git directory can be
 located.
 +
 If the submodule has never been initialized, this is the only place
@@ -137,8 +141,8 @@ directory is automatically moved to `$GIT_DIR/modules/<name>/`
 of the superproject.
 
  * Deinitialized submodule: A `gitlink`, and a `.gitmodules` entry,
-but no submodule working directory. The submodule’s git directory
-may be there as after deinitializing the git directory is kept around.
+but no submodule working directory. The submodule’s Git directory
+may be there as after deinitializing the Git directory is kept around.
 The directory which is supposed to be the working directory is empty instead.
 +
 A submodule can be deinitialized by running `git submodule deinit`.
@@ -160,6 +164,60 @@ from another repository.
 To completely remove a submodule, manually delete
 `$GIT_DIR/modules/<name>/`.
 
+ACTIVE SUBMODULES
+-----------------
+
+A submodule is considered active,
+
+  (a) if `submodule.<name>.active` is set to `true`
+     or
+  (b) if the submodule's path matches the pathspec in `submodule.active`
+     or
+  (c) if `submodule.<name>.url` is set.
+
+and these are evaluated in this order.
+
+For example:
+
+  [submodule "foo"]
+    active = false
+    url = https://example.org/foo
+  [submodule "bar"]
+    active = true
+    url = https://example.org/bar
+  [submodule "baz"]
+    url = https://example.org/baz
+
+In the above config only the submodule 'bar' and 'baz' are active,
+'bar' due to (a) and 'baz' due to (c). 'foo' is inactive because
+(a) takes precedence over (c)
+
+Note that (c) is a historical artefact and will be ignored if the
+(a) and (b) specify that the submodule is not active. In other words,
+if we have an `submodule.<name>.active` set to `false` or if the
+submodule's path is excluded in the pathspec in `submodule.active`, the
+url doesn't matter whether it is present or not. This is illustrated in
+the example that follows.
+
+  [submodule "foo"]
+    active = true
+    url = https://example.org/foo
+  [submodule "bar"]
+    url = https://example.org/bar
+  [submodule "baz"]
+    url = https://example.org/baz
+  [submodule "bob"]
+    ignore = true
+  [submodule]
+    active = b*
+    active = :(exclude) baz
+
+In here all submodules except 'baz' (foo, bar, bob) are active.
+'foo' due to its own active flag and all the others due to the
+submodule active pathspec, which specifies that any submodule
+starting with 'b' except 'baz' are also active, regardless of the
+presence of the .url field.
+
 Workflow for a third party library
 ----------------------------------