Update merge request filter docs

Annabel Dunstone Grayrequested to merge
activity-filters-docs into master

What does this MR do and why?

Adds docs for merge request activity filters (not yet implemented)

Related to #387070 (closed)

MR acceptance checklist

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

Edited by Annabel Dunstone Gray

Merge request reports