Documentation/git-am.txt: mention mailinfo.scissors config variable

It was already documented, but the user had to follow the link to
git-mailinfo.txt to find it.

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

Documentation/git-am.txt

@@ -52,7 +52,8 @@ OPTIONS
 -c::
 --scissors::
 	Remove everything in body before a scissors line (see
-	linkgit:git-mailinfo[1]).
+	linkgit:git-mailinfo[1]). Can be activated by default using
+	the `mailinfo.scissors` configuration variable.
 
 --no-scissors::
 	Ignore scissors lines (see linkgit:git-mailinfo[1]).