Document merge.branchdesc configuration variable

This was part of the "branch description" feature in the larger
"help people communicate better during their pull based workflow"
topic, but was never documented.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-fmt-merge-msg.txt

@@ -53,6 +53,11 @@ OPTIONS
 CONFIGURATION
 -------------
 
+merge.branchdesc::
+	In addition to branch names, populate the log message with
+	the branch description text associated with them.  Defaults
+	to false.
+
 merge.log::
 	In addition to branch names, populate the log message with at
 	most the specified number of one-line descriptions from the