Add docs for burnup feature flag

Copied content from dev docs

app/controllers/groups/milestones_controller.rb

@@ -6,7 +6,7 @@ class Groups::MilestonesController < Groups::ApplicationController
   before_action :milestone, only: [:edit, :show, :update, :issues, :merge_requests, :participants, :labels, :destroy]
   before_action :authorize_admin_milestones!, only: [:edit, :new, :create, :update, :destroy]
   before_action do
-    push_frontend_feature_flag(:burnup_charts, @group)
+    push_frontend_feature_flag(:burnup_charts, @group, default_enabled: true)
   end
 
   feature_category :issue_tracking

app/controllers/projects/milestones_controller.rb

@@ -7,7 +7,7 @@ class Projects::MilestonesController < Projects::ApplicationController
   before_action :check_issuables_available!
   before_action :milestone, only: [:edit, :update, :destroy, :show, :issues, :merge_requests, :participants, :labels, :promote]
   before_action do
-    push_frontend_feature_flag(:burnup_charts, @project)
+    push_frontend_feature_flag(:burnup_charts, @project, default_enabled: true)
   end
 
   # Allow read any milestone

config/feature_flags/development/burnup_charts.yml

 ---
 name: burnup_charts
 introduced_by_url:
-rollout_issue_url:
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/268350
 group: group::project management
 type: development
-default_enabled: false
+default_enabled: true

doc/user/project/milestones/burndown_and_burnup_charts.md

@@ -103,7 +103,14 @@ Reopened issues are considered as having been opened on the day after they were
 
 ## Burnup charts
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/6903) in [GitLab Starter](https://about.gitlab.com/pricing/) 13.5.
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/6903) in [GitLab Starter](https://about.gitlab.com/pricing/) 13.5.
+> - It's [deployed behind a feature flag](../../feature_flags.md), enabled by default.
+> - It's enabled on GitLab.com.
+> - It's recommended for production use.
+> - For GitLab self-managed instances, GitLab administrators can opt to [disable it](#enable-or-disable-burnup-charts). **(STARTER ONLY)**
+
+CAUTION: **Warning:**
+This feature might not be available to you. Check the **version history** note above for details.
 
 Burnup charts show the assigned and completed work for a milestone.
 
@@ -129,6 +136,25 @@ Burnup charts can show either the total number of issues or total weight for eac
 day of the milestone. Use the toggle above the charts to switch between total
 and weight.
 
+### Enable or disable burnup charts **(STARTER ONLY)**
+
+Burnup charts is under development but ready for production use.
+It is deployed behind a feature flag that is **enabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../../administration/feature_flags.md)
+can opt to disable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:burnup_charts)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:burnup_charts)
+```
+
 <!-- ## Troubleshooting
 
 Include any troubleshooting steps that you can foresee. If you know beforehand what issues

ee/app/controllers/groups/iterations_controller.rb

@@ -6,7 +6,7 @@ class Groups::IterationsController < Groups::ApplicationController
   before_action :authorize_create_iteration!, only: [:new, :edit]
   before_action do
     push_frontend_feature_flag(:iteration_charts, group)
-    push_frontend_feature_flag(:burnup_charts, group)
+    push_frontend_feature_flag(:burnup_charts, group, default_enabled: true)
   end
 
   feature_category :issue_tracking

ee/app/controllers/projects/iterations/inherited_controller.rb

@@ -5,7 +5,7 @@ class Projects::Iterations::InheritedController < Projects::ApplicationControlle
   before_action :authorize_show_iteration!
   before_action do
     push_frontend_feature_flag(:iteration_charts, project)
-    push_frontend_feature_flag(:burnup_charts, project)
+    push_frontend_feature_flag(:burnup_charts, project, default_enabled: true)
   end
 
   feature_category :issue_tracking

ee/app/controllers/projects/iterations_controller.rb

@@ -5,7 +5,7 @@ class Projects::IterationsController < Projects::ApplicationController
   before_action :authorize_show_iteration!
   before_action do
     push_frontend_feature_flag(:iteration_charts, project)
-    push_frontend_feature_flag(:burnup_charts, project)
+    push_frontend_feature_flag(:burnup_charts, project, default_enabled: true)
   end
 
   feature_category :issue_tracking

ee/app/models/ee/milestone.rb

@@ -17,7 +17,7 @@ module EE
     alias_method :supports_timebox_charts?, :supports_milestone_charts?
 
     def burnup_charts_available?
-      ::Feature.enabled?(:burnup_charts, resource_parent)
+      ::Feature.enabled?(:burnup_charts, resource_parent, default_enabled: true)
     end
   end
 end

ee/changelogs/unreleased/psi-enable-burnup-charts.yml

+---
+title: Add burnup charts to milestone page
+merge_request: 45477
+author:
+type: added