git-submodule.txt: "--branch <branch>" option defaults to 'master'

This behavior is mentioned in gitmodules.txt but not in
git-submodule.txt so we copy the information over so that it is not
missed.

Also, add the missed argument to the -b/--branch option.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Denton-L

Documentation/git-submodule.txt

@@ -255,13 +255,14 @@ OPTIONS
 	This option is only valid for the deinit command. Unregister all
 	submodules in the working tree.
 
--b::
---branch::
+-b <branch>::
+--branch <branch>::
 	Branch of repository to add as submodule.
 	The name of the branch is recorded as `submodule.<name>.branch` in
 	`.gitmodules` for `update --remote`.  A special value of `.` is used to
 	indicate that the name of the branch in the submodule should be the
-	same name as the current branch in the current repository.
+	same name as the current branch in the current repository.  If the
+	option is not specified, it defaults to 'master'.
 
 -f::
 --force::