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>