Update annotations docs

This MR adds annotations UI screenshots and
removes the mention of feature flag

doc/api/metrics_dashboard_annotations.md

@@ -4,16 +4,6 @@
 
 Metrics dashboard annotations allow you to indicate events on your graphs at a single point in time or over a timespan.
 
-## Enable the metrics dashboard annotations API
-
-The `:metrics_dashboard_annotations` feature flag is disabled by default.
-To turn on this API, ask a GitLab administrator with Rails console
-access to run the following command:
-
-```ruby
-Feature.enable(:metrics_dashboard_annotations)
-```
-
 ## Create a new annotation
 
 ```plaintext

doc/user/project/integrations/prometheus.md

@@ -673,7 +673,8 @@ The options are:
 
 ### Dashboard Annotations
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/211330) in GitLab 12.10 (enabled by feature flag `metrics_dashboard_annotations`).
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/211330) in GitLab 12.10 (enabled by feature flag `metrics_dashboard_annotations`).
+> - [Feature flag removed](https://gitlab.com/gitlab-org/gitlab/-/issues/215224) in GitLab 13.0.
 
 You can use **Metrics Dashboard Annotations** to mark any important events on
 every metrics dashboard by adding annotations to it. While viewing a dashboard,
@@ -685,6 +686,8 @@ its description.
 You can create annotations by making requests to the
 [Metrics dashboard annotations API](../../../api/metrics_dashboard_annotations.md)
 
+![Annotations UI](img/metrics_dashboard_annotations_ui_v13.0.png)
+
 ### View Logs **(ULTIMATE)**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/122013) in GitLab 12.8.