DOC-6282 Added a note about Active-Active memory usage (#2814)

Author: rrelledge

content/operate/rs/references/metrics/resource-usage.md

@@ -65,6 +65,12 @@ Used memory does not include:
 
 Used memory is not measured during [shard migration]({{< relref "/operate/rs/databases/configure/replica-ha" >}}).
 
+Additional factors for [Active-Active databases]({{<relref "/operate/rs/databases/active-active">}}):
+
+- By default, the [Active-Active replication backlog]({{<relref "/operate/rs/databases/active-active/manage#replication-backlog">}}), used for synchronization between clusters or regions, is set to 1% of the database size.
+
+- Active-Active databases have a lower threshold for activating the eviction policy, because it requires propagation to all participating clusters. The eviction policy starts to evict keys when one of the Active-Active instances reaches 80% of its memory limit.
+
 **Components measured**: Database and Shard
 
 ### Free RAM