document merge request approvers over-ride limitations

In the merge request approvers documentation we document how to over-ride the number of approvers for individual merge requests in a project that requires approvers. We do not document that when over-riding the number of approvers you must specify a number greater than or equal to the number of approvers required at the project level.

It seems the app should be protecting against setting a lower value, but this issue also exists.

The customer in https://gitlab.zendesk.com/agent/tickets/128920, running 11.9.12, has a case of a MR failing to merge because it's failing this validation at merge time.