Merge branch 'msj-epics-assignees' into 'master'

Document assignees for epics

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/163621



Merged-by: Brendan Lynch <blynch@gitlab.com>
Approved-by: Kushal Pandya <kushal@gitlab.com>
Approved-by: Brendan Lynch <blynch@gitlab.com>
Co-authored-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>