doc: show usage of branch description

The branch description will be included in 'git format-patch
--cover-letter' and in 'git pull-request' emails. It can also
be used in the automatic merge message. Tell the reader.

While here, clarify that the description may be a multi-line
explanation of the purpose of the branch's patch series.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index c0fd470..037ceb2 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -213,7 +213,7 @@
 
 --[no-]cover-letter::
 	In addition to the patches, generate a cover letter file
-	containing the shortlog and the overall diffstat.  You can
+	containing the branch description, shortlog and the overall diffstat.  You can
 	fill in a description in the file before sending it out.
 
 --notes[=<ref>]::