Iteration board list requires additional feature flag to work

doc/user/project/issue_board.md

@@ -343,10 +343,10 @@ As in other list types, click the trash icon to remove a list.
 ### Iteration lists **(PREMIUM)**
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/250479) in GitLab 13.10.
-> - It's [deployed behind the `board_new_lists` feature flag](../feature_flags.md), disabled by default.
+> - It's [deployed behind the `board_new_lists` and `iteration_board_lists` feature flags](../feature_flags.md), disabled by default.
 > - It's disabled on GitLab.com.
 > - It's recommended for production use.
-> - To use it in GitLab self-managed instances, ask a GitLab administrator to [enable it](#enable-or-disable-new-add-list-form).
+> - To use it in GitLab self-managed instances, ask a GitLab administrator to enable the feature flags: [`board_new_lists`](#enable-or-disable-new-add-list-form) and [`iteration_board_lists`](#enable-or-disable-iteration-lists-in-boards).
 
 WARNING:
 This feature might not be available to you. Check the **version history** note above for details.
@@ -662,3 +662,25 @@ To disable it:
 ```ruby
 Feature.disable(:board_new_list)
 ```
+
+### Enable or disable iteration lists in boards **(PREMIUM SELF)**
+
+NOTE:
+To enable iteration lists in boards, you also need to enable the [new add list form](#enable-or-disable-new-add-list-form).
+
+The iteration list is under development and not ready for production use. It is
+deployed behind a feature flag that is **disabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../administration/feature_flags.md)
+can enable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:iteration_board_lists)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:iteration_board_lists)
+```