Improve documentation concerning the status.submodulesummary setting

'git status' and 'git commit' can be told to also show the output of "git
submodule summary" by setting the "status.submodulesummary" config option.
But status and commit also honor the "diff.ignoreSubmodules" and the
"submodule.<name>.ignore" settings, which then disable the summary partly
or completely. This - and the fact that the last two settings do not
affect the "git submodule" commands at all - is not well documented.

Extend the documentation in those places where "status.submodulesummary",
"diff.ignoreSubmodules" and "submodule.<name>.ignore" are described to
better explain these dependencies.

Thanks-to: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>