Documentation/config.txt: document mailinfo.scissors

Documentation/config.txt: document mailinfo.scissors

The variable was documented in git-mailinfo.txt, but not in config.txt.
The detailed documentation is still the one of --scissors in
git-mailinfo.txt, but we give enough information here to let the user
understand what it is about, and to make it easy to find it (e.g.
searching ">8" and "8<" finds it).

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>