Merge branch 'ff-dev-unused-flags-warning' into 'master'

[Docs] Add warning about unused feature flags to dev docs

See merge request gitlab-org/gitlab!48049

doc/development/feature_flags/development.md

@@ -23,6 +23,12 @@ All newly-introduced feature flags should be [disabled by default](process.md#fe
 NOTE: **Note:**
 This document is the subject of continued work as part of an epic to [improve internal usage of Feature Flags](https://gitlab.com/groups/gitlab-org/-/epics/3551). Raise any suggestions as new issues and attach them to the epic.
 
+## Risk of a broken master (main) branch
+
+Feature flags **must** be used in the MR that introduces them. Not doing so causes a
+[broken master](https://about.gitlab.com/handbook/engineering/workflow/#broken-master) scenario due
+to the `rspec:feature-flags` job that only runs on the `master` branch.
+
 ## Types of feature flags
 
 Choose a feature flag type that matches the expected usage.