Add note about Keep divergent refs option

What

Note that the Keep divergent refs option should be selected when mirroring protected branches only.

Why

While manually re-creating mirroring rules on 24th April, we encountered a problem where mirroring was attempting to delete branches on the security mirror (gitlab-org/security/gitlab) even though the branches could be fast-forwarded - #21066 (comment 2467422762).

With the Keep divergent refs option selected, mirroring will complain about branches that have diverged, allowing us to determine the next steps, rather than attempting to delete the branches. Hopefully, this should also cause mirroring to fast-forward branches that can be fast-forwarded.