Increase visibility that merge request approval rules are not retrospective

From a comment:

There are only two instances where I've seen a rule not show up before:

The MR was created before the rule was created and the new rule isn't applied to the MR (cos' reasons 🤷)

If you add a new approval rule it might not affect your current merge requests. The visibility of that behaviour should be increased when you add a new approval rule on this page: