submodules doc: update documentation for "." used for submodule branches

4d7bc52b17 ("submodule update: allow '.' for branch value",
2016-08-03) adopted from Gerrit a feature to set "." as a special
value of "submodule.<name>.branch" in .gitmodules file to indicate
that the tracking branch in the submodule should be the same as the
current branch in the superproject.

Update the documentation to describe this.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -259,7 +259,9 @@ OPTIONS
 --branch::
 	Branch of repository to add as submodule.
 	The name of the branch is recorded as `submodule.<name>.branch` in
-	`.gitmodules` for `update --remote`.
+	`.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.
 
 -f::
 --force::

Documentation/gitmodules.txt

@@ -50,8 +50,11 @@ submodule.<name>.update::
 
 submodule.<name>.branch::
 	A remote branch name for tracking updates in the upstream submodule.
-	If the option is not specified, it defaults to 'master'.  See the
-	`--remote` documentation in linkgit:git-submodule[1] for details.
+	If the option is not specified, it defaults to 'master'.  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.  See the `--remote` documentation in
+	linkgit:git-submodule[1] for details.
 
 submodule.<name>.fetchRecurseSubmodules::
 	This option can be used to control recursive fetching of this