doc: preparatory clean-up of description on the sign-off option

Almost identical text on the signed-off-by trailer appears in the
documentation for "git commit" and "git merge" and its friends.

Introduce a new signoff-option.txt file to be shared.  A couple of
things of note are:

 - The short-form "-s" is available only in "git commit", but not in
   commands that are friends of "git merge", as it is used as a
   short-hand for "--strategy".

 - The original lacks description on the negated "--no-signoff" form
   on "git commit" side, but it equally is applicable.  It however
   was unclear in the original text that not adding a Signed-off-by
   trailer is the default, so rephrase to explain it as a way to
   countermand a --signoff option that appeared earlier on the same
   command line.

This is in preparation to apply a further clarification on what
exactly the Signed-off-by trailer means.

Suggested-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Bradley M. Kuhn <bkuhn@sfconservancy.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -59,6 +59,7 @@ commit by giving the same set of parameters (options and paths).
 If you make a commit and then find a mistake immediately after
 that, you can recover from it with 'git reset'.
 
+:git-commit: 1
 
 OPTIONS
 -------
@@ -163,14 +164,7 @@ The `-m` option is mutually exclusive with `-c`, `-C`, and `-F`.
 	message, the commit is aborted.  This has no effect when a message
 	is given by other means, e.g. with the `-m` or `-F` options.
 
--s::
---signoff::
-	Add Signed-off-by line by the committer at the end of the commit
-	log message.  The meaning of a signoff depends on the project,
-	but it typically certifies that committer has
-	the rights to submit this work under the same license and
-	agrees to a Developer Certificate of Origin
-	(see http://developercertificate.org/ for more information).
+include::signoff-option.txt[]
 
 -n::
 --no-verify::

Documentation/merge-options.txt

@@ -77,16 +77,7 @@ When not possible, refuse to merge and exit with a non-zero status.
 With --no-log do not list one-line descriptions from the
 actual commits being merged.
 
---signoff::
---no-signoff::
-	Add Signed-off-by line by the committer at the end of the commit
-	log message.  The meaning of a signoff depends on the project,
-	but it typically certifies that committer has
-	the rights to submit this work under the same license and
-	agrees to a Developer Certificate of Origin
-	(see http://developercertificate.org/ for more information).
-+
-With --no-signoff do not add a Signed-off-by line.
+include::signoff-option.txt[]
 
 --stat::
 -n::

Documentation/signoff-option.txt

@@ -0,0 +1,14 @@
+ifdef::git-commit[]
+-s::
+endif::git-commit[]
+--signoff::
+--no-signoff::
+	Add Signed-off-by line by the committer at the end of the commit
+	log message.  The meaning of a signoff depends on the project,
+	but it typically certifies that committer has
+	the rights to submit this work under the same license and
+	agrees to a Developer Certificate of Origin
+	(see http://developercertificate.org/ for more information).
++
+The --no-signoff option can be used to countermand an earlier --signoff
+option on the command line.