Add documentation for runner GIT_SUBMODULE_UPDATE_FLAGS

What does this MR do?

This is a companion MR adding documentation for the implementation of GIT_SUBMODULE_UPDATE_FLAGS in Add support for extra submodule update flags (gitlab-runner!3192 - merged).

I based this off the GIT_FETCH_EXTRA_FLAGS section just above this one.

@ajwalker you may want to have a quick look at this as we already discussed the flag overlap in the implementation MR. Luckily, a similar situation happens with the fetch extra flags, so I was able to document it mostly the same way.

Related issues

gitlab CI not checking out latest submodule's c... (gitlab-runner#3011 - closed),
Run git submodule update --force (gitlab-runner#27277 - closed).

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