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

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: W. Trevor King <wking@tremily.us>
Reviewed-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>