Remove warning from Instance Statistics doc

Previously an MR added Instance Statistics documentation to describe
the feature flag and a warning about availability. The feature flag
has since been set to default_enable and the feature flag description
in the documentation was removed. This finishes the removal by
removing the warning.

doc/user/admin_area/analytics/instance_statistics.md

@@ -6,7 +6,10 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 # Instance Statistics **(CORE)**
 
-> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/235754) in GitLab 13.4.
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/235754) in GitLab 13.5 behind a feature flag, disabled by default.
+> - [Became enabled by default](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46962) in GitLab 13.6.
+> - It's enabled on GitLab.com.
+> - It's recommended for production use.
 
 CAUTION: **Warning:**
 This feature might not be available to you. Check the **version history** note above for details.
@@ -36,3 +39,22 @@ in the categories shown in [Total counts](#total-counts).
 These charts help you visualize how rapidly these records are being created on your instance.
 
 ![Instance Activity Pipelines chart](img/instance_activity_pipelines_chart_v13_6.png)
+
+### Enable or disable Instance Statistics
+
+In GitLab version 13.5 only, Instance Statistics was under development and not ready for production use.
+It was deployed behind a feature flag that was **disabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../../administration/feature_flags.md)
+can opt to enable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:instance_statistics)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:instance_statistics)
+```