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

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>