Documentation: explain optional arguments better

Improve the documentation of commands taking optional arguments in two
ways:

* Documents the behavior of '-O' (for grep) and '-S' (for commands
  creating commits) when used without the optional argument.

* Document the syntax of these options.

For the second point, the behavior is documented in gitcli(7), but it is
easy for users to miss, and hard for the same user to understand why e.g.
"git status -u no" does not work.

Document this explicitly in the documentation of each short option having
an optional argument: they are the most error prone since there is no '='
sign between the option and its argument.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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