Update issue boards doc

Editing title in the group issue sidebar became possible
(https://gitlab.com/gitlab-org/gitlab/-/issues/285074).

doc/user/project/issue_board.md

@@ -277,6 +277,32 @@ group and its descendant subgroups. Similarly, you can only filter by group labe
 boards. When updating milestones and labels for an issue through the sidebar update mechanism, again only
 group-level objects are available.
 
+#### GraphQL-based sidebar for group issue boards **(PREMIUM)**
+
+<!-- When the feature flag is removed, integrate this section into the above ("Group issue boards"). -->
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/285074) 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-graphql-based-sidebar-for-group-issue-boards). **(PREMIUM SELF)**
+
+WARNING:
+This feature might not be available to you. Check the **version history** note above for details.
+
+The work-in-progress GraphQL-based sidebar for group issue boards brings better performance and the
+ability to edit issue titles in the issue sidebar.
+
+To **edit an issue's title** in the issue sidebar:
+
+1. In a group issue board, select the issue card. The issue sidebar opens on the right.
+1. Next to the issue's title, select **Edit**.
+
+This is work in progress as of GitLab 13.9. Learn more about the known issues in
+[MR 51480](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51480).
+
+<!-- Add this at the end of the file -->
+
 ### Assignee lists **(PREMIUM)**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/5784) in [GitLab Premium](https://about.gitlab.com/pricing/) 11.0.
@@ -456,24 +482,6 @@ the list by filtering by the following:
 - Release
 - Weight
 
-#### Enable or disable adding issues to the list **(FREE SELF)**
-
-Adding issues to the list 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(:add_issues_button)
-```
-
-To disable it:
-
-```ruby
-Feature.disable(:add_issues_button)
-```
-
 ### Remove an issue from a list
 
 Removing an issue from a list can be done by clicking the issue card and then
@@ -593,3 +601,40 @@ A few things to remember:
 - For performance and visibility reasons, each list shows the first 20 issues
   by default. If you have more than 20 issues, start scrolling down and the next
   20 appear.
+
+## Enable or disable GraphQL-based sidebar for group issue boards **(PREMIUM SELF)**
+
+GraphQL-based sidebar for group issue boards 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(:graphql_board_lists)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:graphql_board_lists)
+```
+
+## Enable or disable adding issues to the list **(FREE SELF)**
+
+Adding issues to the list 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(:add_issues_button)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:add_issues_button)
+```