Add documentation for maintainer only reviews

What does this MR do and why?

We recently added support for a new emoji status, Ⓜ which will mean that the roulette will only suggest team members for reviews on projects they are a maintainer of

This MR adds docs to explain as much

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.