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

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>
Reviewed-by: Taylor Blau <redacted>
Reviewed-by: Bradley M. Kuhn <redacted>
Signed-off-by: Junio C Hamano <redacted>