Follow-up from "New user signup with/without reCAPTCHA A/B test"

The following discussion from !14019 (merged) should be addressed:

• @jameslopez started a discussion: (+4 comments)

  Thanks @ashmckenzie - Looks good! Just left some minor suggestions.

  Do we need any documentation about this? (even if it's internal for us)

I feel we can simply use https://gitlab.com/gitlab-org/gitlab-ee/blob/master/doc/development/rolling_out_changes_using_feature_flags.md#rolling-out-changes