Add more information in the User cohort documentation

We needed to clarify what inactive users are.

doc/user/admin_area/user_cohorts.md

@@ -13,10 +13,14 @@ user cohorts.
 
 ![User cohort example](img/cohorts.png)
 
-For the cohort of June 2016, 163 users have been created on this server. One
-month later, in July 2016, 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.
+For the cohort of June 2016, 163 users have been added on this server and have
+actually been active during 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