Update user-classes.md

As per Engineering feedback at https://pagerduty.slack.com/archives/C01BZFSMQH4/p1754604082617559?thread_ts=1754569692.797319&cid=C01BZFSMQH4

Author: brmdias

docs/manual/user-management/user-classes.md

@@ -70,6 +70,8 @@ There is also a new section in the [System Report](/manual/system-report.md) (On
 * _FullUser.assigned.max:_ Max Count of User Class assignments for FullUser.
 * _FullUser.mapping.current:_ Current Mappings for FullUser. (shown as array of usernames)
 
+The time period used to calculate these counters begins from the date the license is issued.
+
 This section is also included in the Export Report and may be asked for as part of licensing validation.
 
 ## Licensing