Add documentation for lock_on_merge capability
Add some documentation for the lock_on_merge capability, which is currently behind the enforce_locked_labels_on_merge :ops feature flag. This is, at the time of opening, a GitLab only capability.
Implementation guide
Add a new section to the labels documentation, at the bottom, above "Related topics".
Use patterns from:
- Document features behind a feature flag + explain this is available only in GitLab Inc-managed groups and projects.
- Documenting Beta/Experiment features
Related links
UI added in Add UI for lock on merged Merge requests (!129151 - merged)
Relevant issue https://gitlab.com/gitlab-org/gitlab/-/issues/408676+
Relevant comment: !129151 (comment 1515405310)
Edited by Marcin Sedlak-Jakubowski