Document namespace querying

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/128689



Merged-by: Lorena Ciutacu <lciutacu@gitlab.com>
Approved-by: Lorena Ciutacu <lciutacu@gitlab.com>
Reviewed-by: Lorena Ciutacu <lciutacu@gitlab.com>
Co-authored-by: Alex Pooley <apooley@gitlab.com>