Merge branch 'docs-feature-flags-dev' into 'master'

Docs: Clarify that every FF must be documented

See merge request gitlab-org/gitlab!55889

doc/development/documentation/feature_flags.md

@@ -14,6 +14,10 @@ feature flag depends on its state (enabled or disabled). When the state
 changes, the developer who made the change **must update the documentation**
 accordingly.
 
+Every feature introduced to the codebase, even if it's behind a feature flag,
+must be documented. For context, see the
+[latest merge request that updated this guideline](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/47917#note_459984428).
+
 ## Criteria
 
 According to the process of [deploying GitLab features behind feature flags](../feature_flags/process.md):