Merge branch 'user-cohort-better-doc' into 'master'

Add documentation for inactive users in the User cohort

Closes #31195

See merge request !10809

doc/user/admin_area/user_cohorts.md

+# Cohorts
+
+> **Notes:**
+> [Introduced][ce-23361] in GitLab 9.1.
+
+As a benefit of having the [usage ping active](settings/usage_statistics.md),
+GitLab lets you analyze the users' activities of your GitLab installation.
+Under `/admin/cohorts`, when the usage ping is active, GitLab will show the
+monthly cohorts of new users and their activities over time.
+
+## Overview
+
+How do we read the user cohorts table? Let's take an example with the following
+user cohorts.
+
+![User cohort example](img/cohorts.png)
+
+For the cohort of June 2016, 163 users have been added on this server and have
+been active since this month. One month later, in July 2016, out of
+these 163 users, 155 users (or 95% of the June cohort) are still active. Two
+months later, 139 users (or 85%) are still active. 9 months later, we can see
+that only 6% of this cohort are still active.
+
+The Inactive users column shows the number of users who have been added during
+the month, but who have never actually had any activity in the instance.
+
+How do we measure the activity of users? GitLab considers a user active if:
+
+* the user signs in
+* the user has Git activity (whether push or pull).
+
+## Setup
+
+1. [Activate the usage ping](settings/usage_statistics.md)
+2. Go to `/admin/cohorts` to see the user cohorts of the server
+
+[ce-23361]: https://gitlab.com/gitlab-org/gitlab-ce/issues/23361