Add documentation on Usage statistics and link from the admin area

app/views/admin/application_settings/_form.html.haml

@@ -492,6 +492,7 @@
           = f.label :usage_ping_enabled do
             = f.check_box :usage_ping_enabled
             Usage ping enabled
+            = link_to icon('question-circle'), help_page_path("user/admin_area/settings/usage_statistics", anchor: "usage-data")
           .container
             .help-block
               Every week GitLab will report license usage back to GitLab, Inc.

doc/user/admin_area/settings/usage_statistics.md

+# Usage statistics
+
+GitLab Inc. will periodically collect information about your instance in order
+to perform various actions.
+
+All statistics are opt-in and you can always disable them from the admin panel.
+
+## Version check
+
+GitLab can inform you when an update is available and the importance of it.
+
+No information other than the GitLab version and the instance's domain name
+are collected.
+
+In the **Overview** tab you can see if your GitLab version is up to date. There
+are three cases: 1) you are up to date (green), 2) there is an update available
+(yellow) and 3) your version is vulnerable and a security fix is released (red).
+
+In any case, you will see a message informing you of the state and the
+importance of the update.
+
+If enabled, the version status will also be shown in the help page (`/help`)
+for all signed in users.
+
+## Usage data
+
+> [Introduced][ee-557] in GitLab Enterprise Edition 8.10. More statistics
+[were added][ee-735] in GitLab Enterprise Edition 8.12.
+
+GitLab Inc. can collect non-sensitive information about how Enterprise Edition
+customers use their GitLab instance upon the activation of a ping feature
+located in the admin panel (`/admin/application_settings`).
+
+You can see the **exact** JSON payload that your instance sends to GitLab Inc.
+in the "Usage statistics" section of the admin panel.
+
+Nothing qualitative is collected. Only quantitative. Meaning, no project name,
+author name, nature of comments, name of labels, etc.
+
+This is done mainly for the following reasons:
+
+- to have a better understanding on how our users use our product
+- to provide more tools for the customer success team to help customers onboard
+  better.
+
+The total number of the following is sent back to GitLab Inc.:
+
+- Comments
+- Groups
+- Users
+- Projects
+- Issues
+- Labels
+- CI builds
+- Snippets
+- Milestones
+- Todos
+- Pushes
+- Merge requests
+- Environments
+- Triggers
+- Deploy keys
+- Pages
+- Project Services
+- Issue Boards
+- CI Runners
+- Deployments
+- Geo Nodes
+- LDAP Groups
+- LDAP Keys
+- LDAP Users
+- LFS objects
+- Protected branches
+- Releases
+- Remote mirrors
+- Web hooks
+
+## Privacy policy
+
+GitLab Inc. does **not** collect any sensitive information, like project names
+or the content of the comments. GitLab Inc. does not disclose or otherwise make
+available any of the data collected on a customer specific basis.
+
+Read more in about the [Privacy policy](https://about.gitlab.com/privacy).
+
+[ee-557]: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/557
+[ee-735]: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/735