Merge branch 'sn/diff-doc'

* sn/diff-doc:
  docs: clarify git diff modes of operation
  diff,difftool: Don't use the {0,2} notation in usage strings
  CodingGuidelines: Add a section on writing documentation

Documentation/CodingGuidelines

@@ -143,3 +143,55 @@ For C programs:
 
  - When we pass <string, length> pair to functions, we should try to
    pass them in that order.
+
+Writing Documentation:
+
+ Every user-visible change should be reflected in the documentation.
+ The same general rule as for code applies -- imitate the existing
+ conventions.  A few commented examples follow to provide reference
+ when writing or modifying command usage strings and synopsis sections
+ in the manual pages:
+
+ Placeholders are enclosed in angle brackets:
+   <file>
+   --sort=<key>
+   --abbrev[=<n>]
+
+ Possibility of multiple occurences is indicated by three dots:
+   <file>...
+   (One or more of <file>.)
+
+ Optional parts are enclosed in square brackets:
+   [<extra>]
+   (Zero or one <extra>.)
+
+   --exec-path[=<path>]
+   (Option with an optional argument.  Note that the "=" is inside the
+   brackets.)
+
+   [<patch>...]
+   (Zero or more of <patch>.  Note that the dots are inside, not
+   outside the brackets.)
+
+ Multiple alternatives are indicated with vertical bar:
+   [-q | --quiet]
+   [--utf8 | --no-utf8]
+
+ Parentheses are used for grouping:
+   [(<rev>|<range>)...]
+   (Any number of either <rev> or <range>.  Parens are needed to make
+   it clear that "..." pertains to both <rev> and <range>.)
+
+   [(-p <parent>)...]
+   (Any number of option -p, each with one <parent> argument.)
+
+   git remote set-head <name> (-a | -d | <branch>)
+   (One and only one of "-a", "-d" or "<branch>" _must_ (no square
+   brackets) be provided.)
+
+ And a somewhat more contrived example:
+   --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
+   Here "=" is outside the brackets, because "--diff-filter=" is a
+   valid usage.  "*" has its own pair of brackets, because it can
+   (optionally) be specified only when one or more of the letters is
+   also provided.

Documentation/git-diff.txt

@@ -8,12 +8,17 @@ git-diff - Show changes between commits, commit and working tree, etc
 
 SYNOPSIS
 --------
-'git diff' [<common diff options>] <commit>{0,2} [--] [<path>...]
+[verse]
+'git diff' [options] [<commit>] [--] [<path>...]
+'git diff' [options] --cached [<commit>] [--] [<path>...]
+'git diff' [options] <commit> <commit> [--] [<path>...]
+'git diff' [options] [--no-index] [--] <path> <path>
 
 DESCRIPTION
 -----------
-Show changes between two trees, a tree and the working tree, a
-tree and the index file, or the index file and the working tree.
+Show changes between the working tree and the index or a tree, changes
+between the index and a tree, changes between two trees, or changes
+between two files on disk.
 
 'git diff' [--options] [--] [<path>...]::
 

Documentation/git-difftool.txt

@@ -7,13 +7,14 @@ git-difftool - Show changes using common diff tools
 
 SYNOPSIS
 --------
-'git difftool' [<options>] <commit>{0,2} [--] [<path>...]
+'git difftool' [<options>] [<commit> [<commit>]] [--] [<path>...]
 
 DESCRIPTION
 -----------
 'git difftool' is a git command that allows you to compare and edit files
 between revisions using common diff tools.  'git difftool' is a frontend
-to 'git diff' and accepts the same options and arguments.
+to 'git diff' and accepts the same options and arguments. See
+linkgit:git-diff[1].
 
 OPTIONS
 -------

builtin/diff.c

@@ -22,7 +22,7 @@ struct blobinfo {
 };
 
 static const char builtin_diff_usage[] =
-"git diff <options> <rev>{0,2} -- <path>*";
+"git diff [<options>] [<commit> [<commit>]] [--] [<path>...]";
 
 static void stuff_change(struct diff_options *opt,
 			 unsigned old_mode, unsigned new_mode,