Documentation/config: clarify the meaning of submodule.<name>.update

Documentation/config: clarify the meaning of submodule.<name>.update

With more commands (that potentially change a submodule) paying attention
to submodules as well as the recent discussion[1] on
submodule.<name>.update, let's spell out that submodule.<name>.update
is strictly to be used for configuring the "submodule update" command
and not to be obeyed by other commands.

These other commands usually have a strict meaning of what they should
do (i.e. checkout, reset, rebase, merge) as well as have their name
overlapping with the modes possible for submodule.<name>.update.

[1] https://public-inbox.org/git/4283F0B0-BC1C-4ED1-8126-7E512D84484B@gmail.com/
    submodule.<name>.update was set to "none", triggering unexpected
    behavior as the submodule was thought to never be touched.
    However a newer version of Git taught 'git pull --rebase' to also
    populate and rebase submodules if they were active.
    The newer options such as submodule.active and command specific
    flags would not have triggered unexpected behavior.

Reported-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>