Update docs for retarget MRs

This ensures that:
- it is ready for production use
- documents `remove source branch` caveat

doc/user/project/merge_requests/getting_started.md

@@ -207,9 +207,9 @@ is set for deletion, the merge request widget displays the
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/320902) in GitLab 13.9.
 > - It's [deployed behind a feature flag](../../feature_flags.md), disabled by default.
-> - It's disabled on GitLab.com.
-> - It's not recommended for production use.
-> - To use it in GitLab self-managed instances, ask a GitLab administrator to [enable it](#enable-or-disable-branch-retargeting-on-merge).
+> - [Became enabled by default](https://gitlab.com/gitlab-org/gitlab/-/issues/320895) on GitLab 13.10.
+> - It's recommended for production use.
+> - To use it in GitLab self-managed instances, ask a GitLab administrator to [disable it](#enable-or-disable-branch-retargeting-on-merge). **(FREE SELF)**
 
 In specific circumstances, GitLab can retarget the destination branch of
 open merge request, if the destination branch merges while the merge request is
@@ -230,6 +230,12 @@ GitLab retargets up to four merge requests when their target branch is merged in
 `master`, so you don't need to perform this operation manually. Merge requests from
 forks are not retargeted.
 
+The feature today works only one a merge. Clicking the `Remove source branch` button
+after the merge request was merged will not automatically retarget merge request.
+The feature today works only on merge. Clicking the **Remove source branch** button
+after the merge request was merged will not automatically retarget a merge request.
+This improvement is [tracked as a follow-up](https://gitlab.com/gitlab-org/gitlab/-/issues/321559).
+
 ## Recommendations and best practices for Merge Requests
 
 - When working locally in your branch, add multiple commits and only push when