Update docs for issue boards

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/153357



Merged-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Approved-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Reviewed-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Co-authored-by: Chad Lavimoniere <clavimoniere@gitlab.com>