Merge branch 'docs-activity-filter-has-mr' into 'master'

Update doc for the activity filter

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/135966



Merged-by: Russell Dickenson <rdickenson@gitlab.com>
Approved-by: Russell Dickenson <rdickenson@gitlab.com>
Co-authored-by: Samantha Ming <sming@gitlab.com>