Improve documentation on Coverage-Check approval rule

Aim

To improve documentation on the Coverage-Check approval . There was some confusion in recent months within the support team when a project had the Coverage-Check rule enabled and a pipeline with no coverage report but still required approval.

The expected outcome is to update the documentation to make this clear, in an attempt to potentially deflect support tickets, help the support engineering team and reduce potential future RFHs related to this.

This was clarified by the pipeline execution team in the below RFHs.

https://gitlab.com/gitlab-com/request-for-help/-/issues/3172

https://gitlab.com/gitlab-com/request-for-help/-/issues/3111