Update group sharing access for service accounts

What does this MR do and why?

References

Based on the discussion in https://gitlab.com/gitlab-org/gitlab/-/issues/579877#note_2939722985, we have disable direct invites for composite ID service accounts, however they can still be granted membership intentionally by a group owner should group sharing. This note informs user about that additional consideration.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/579877

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