Merge branch 'docs-feature-flag-user-list-strategy' into 'master'

Document new User List Strategy

Closes #35930

See merge request gitlab-org/gitlab!34526

doc/user/project/operations/feature_flags.md

@@ -146,6 +146,14 @@ CAUTION: **Caution:**
 The Unleash client **must** be given a user ID for the feature to be enabled for
 target users. See the [Ruby example](#ruby-application-example) below.
 
+#### List
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/35930) in GitLab 13.1.
+
+A feature flag can be enabled for lists of users created with the [Feature Flag User List API](../../../api/feature_flag_user_lists.md).
+Similar to [User IDs](#user-ids), it uses the Unleash [`userWithId`](https://unleash.github.io/docs/activation_strategy#userwithid)
+activation strategy.
+
 ## Integrate feature flags with your application
 
 To use feature flags with your application, get access credentials from GitLab.