Documentation: branch: update --merged description

Update the documentation for --merged and --no-merged to explain
the meaning of the optional parameter introduced in commit 049716b
(branch --merged/--no-merged: allow specifying arbitrary commit,
2008-07-08).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-branch.txt

@@ -134,11 +134,13 @@ start-point is either a local or remote branch.
 --contains <commit>::
 	Only list branches which contain the specified commit.
 
---merged::
-	Only list branches which are fully contained by HEAD.
+--merged [<commit>]::
+	Only list branches whose tips are reachable from the
+	specified commit (HEAD if not specified).
 
---no-merged::
-	Do not list branches which are fully contained by HEAD.
+--no-merged [<commit>]::
+	Only list branches whose tips are not reachable from the
+	specified commit (HEAD if not specified).
 
 <branchname>::
 	The name of the branch to create or delete.