Merge branch 'ld-docs-feature-flag-mixing-actors' into 'master'

Document when mixing flag actors is safe

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/165466



Merged-by: Kamil Trzciński <ayufan@ayufan.eu>
Approved-by: Kamil Trzciński <ayufan@ayufan.eu>
Reviewed-by: Kamil Trzciński <ayufan@ayufan.eu>
Co-authored-by: Luke Duncalfe <lduncalfe@gitlab.com>
Co-authored-by: Luke Duncalfe <lduncalfe@eml.cc>