docs: correct the seat-overage claim on membership reactivation

Nothing bills for the extra seat: `add_single_seat` is only called from
the invite flow, and the codebase's overage billing covers API calls, not
seats. An organisation reactivated past its limit simply sits over it.

Ref: #8368

Author: khvn26

api/users/models.py

@@ -280,8 +280,11 @@ def set_organisation_membership_active(
 
         Reactivation deliberately does not enforce the plan's seat limit: this is
         driven by an external identity provider over SCIM, where failing the call
-        would leave the provider and Flagsmith out of sync. Going over the limit
-        is handled by the usual seat overage billing instead.
+        would leave the provider and Flagsmith out of sync.
+
+        Note that nothing then charges for the extra seat — `add_single_seat` is
+        only called when joining from an invite — so an organisation can sit over
+        its limit until someone acts on it.
         """
         user_organisation = UserOrganisation.objects.get(
             user=self, organisation=organisation