Merge branch 'docs-add-note-to-features-api' into 'master'

Add Introduction Text to the Features API Documentation

See merge request gitlab-org/gitlab!31988

doc/api/features.md

 # Features flags API
 
+This API is for managing Flipper-based [feature flags used in development of GitLab](../development/feature_flags/index.md).
+
 All methods require administrator authorization.
 
 Notice that currently the API only supports boolean and percentage-of-time gate