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>

1 files changed, 14 insertions, 0 deletions

diff --git a/Documentation/signoff-option.txt b/Documentation/signoff-option.txt
new file mode 100644
index 0000000..d1c6713
--- /dev/null
+++ b/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.