Document moving issue tasks behind feature flag

doc/user/project/issues/managing_issues.md

@@ -131,6 +131,21 @@ To move an issue:
 1. Search for a project to move the issue to.
 1. Select **Move**.
 
+### Moving tasks when the parent issue is moved
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/371252) in GitLab 16.9 [with a flag](../../../administration/feature_flags.md) named `move_issue_children`. Disabled by default.
+
+FLAG:
+On self-managed GitLab, by default this feature is not available. To make it available per project or for your entire instance,
+an administrator can [enable the feature flag](../../../administration/feature_flags.md) named `move_issue_children`.
+On GitLab.com and GitLab Dedicated, this feature is not available.
+The feature is not ready for production use.
+
+When this feature is enabled, when you move an issue to another project, all its child tasks are also
+moved to the target project and remain associated as child tasks on the moved issue.
+Each task is moved the same way as the parent, that is, it's closed in the original project and
+copied to the target project.
+
 ### Bulk move issues
 
 DETAILS: