submodule documentation: add options to the subcommand

submodule documentation: add options to the subcommand

When reading up on a subcommand of `git submodule <subcommand>`,
it is convenient to have its options nearby and not just at the
top of the man page.  Add the options to each subcommand.

While at it, also document the `--checkout` option for `update`.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>