Documentation/gitmodules: Only 'update' and 'url' are required

Descriptions for all the settings fell under the initial "Each
submodule section also contains the following required keys:".  The
example shows sections with just 'path' and 'url' entries, which are
indeed required, but we should still make the required/optional
distinction explicit to clarify that the rest of them are optional.
Signed-off-by: NW. Trevor King <wking@tremily.us>
Reviewed-by: NHeiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/gitmodules.txt

@@ -35,6 +35,8 @@ submodule.<name>.url::
 	linkgit:git-clone[1] or (if it begins with ./ or ../) a location
 	relative to the superproject's origin repository.
 
+In addition, there are a number of optional keys:
+
 submodule.<name>.update::
 	Defines what to do when the submodule is updated by the superproject.
 	If 'checkout' (the default), the new commit specified in the