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>

Author: Philip Oakley

Documentation/git-branch.txt

@@ -193,7 +193,9 @@ start-point is either a local or remote-tracking branch.
 
 --edit-description::
 	Open an editor and edit the text to explain what the branch is
-	for, to be used by various other commands (e.g. `request-pull`).
+	for, to be used by various other commands (e.g. `format-patch`,
+	`request-pull`, and `merge` (if enabled)). Multi-line explanations
+	may be used.
 
 --contains [<commit>]::
 	Only list branches which contain the specified commit (HEAD

Documentation/git-format-patch.txt

@@ -213,7 +213,7 @@ feeding the result to `git send-email`.
 
 --[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>]::

Documentation/git-merge.txt

@@ -78,7 +78,7 @@ will be appended to the specified message.
 +
 The 'git fmt-merge-msg' command can be
 used to give a good default for automated 'git merge'
-invocations.
+invocations. The automated message can include the branch description.
 
 --[no-]rerere-autoupdate::
 	Allow the rerere mechanism to update the index with the

Documentation/git-request-pull.txt

@@ -14,7 +14,8 @@ DESCRIPTION
 -----------
 
 Generate a request asking your upstream project to pull changes into
-their tree.  The request, printed to the standard output, summarizes
+their tree.  The request, printed to the standard output,
+begins with the branch description, summarizes
 the changes and indicates from where they can be pulled.
 
 The upstream project is expected to have the commit named by