submodule: improve documentation of update subcommand

The documentation of 'git submodule update' has several problems:

1) It mentions that value 'none' of submodule.$name.update can be
   overridden by --checkout, but other combinations of configuration
   values and command line options are not mentioned.

2) The documentation of submodule.$name.update is scattered across three
   places, which is confusing.

3) The documentation of submodule.$name.update in gitmodules.txt is
   incorrect, because the code always uses the value from .git/config
   and never from .gitmodules.

4) Documentation of --force was incomplete, because it is only effective
   in case of checkout method of update.

Fix all these problems by documenting submodule.*.update in
git-submodule.txt and make everybody else refer to it.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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