Documentation/submodule: add command references and update options

Documentation/submodule: add command references and update options

Reference the "git diff" and "git status" commands where they learned
functionality that in earlier git versions was only available through the
'summary' and 'status' subcommands of "git submodule".

The short option '-n' for '--summary-limit' was missing from the synopsis
and the --init option was missing from the "options" section, add those
there. And while at it, quote all options in backticks so they are
decorated properly in the output formats which support that.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>