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>

Documentation/config.txt

@@ -1699,6 +1699,13 @@ log.mailmap::
 	If true, makes linkgit:git-log[1], linkgit:git-show[1], and
 	linkgit:git-whatchanged[1] assume `--use-mailmap`.
 
+mailinfo.scissors::
+	If true, makes linkgit:git-mailinfo[1] (and therefore
+	linkgit:git-am[1]) act by default as if the --scissors option
+	was provided on the command-line. When active, this features
+	removes everything from the message body before a scissors
+	line (i.e. consisting mainly of ">8", "8<" and "-").
+
 mailmap.file::
 	The location of an augmenting mailmap file. The default
 	mailmap, located in the root of the repository, is loaded