OBSDOCS-765: Add clarification about exceeded maximum retention size to docs

Author: eromanova97

modules/monitoring-modifying-retention-time-and-size-for-prometheus-metrics-data.adoc

@@ -28,6 +28,11 @@ If any data blocks exceed the defined retention time or the defined size limit,
 * If you do not define a value for `retentionSize` and only define a value for `retention`, only the `retention` value applies.
 * If you set the `retentionSize` or `retention` value to number 0, the default settings apply. The default settings set retention time to 15 days and do not set a value for retention size.
 
+[NOTE]
+====
+Data compaction occurs every two hours. Therefore, a persistent volume (PV) might fill up before compaction, potentially exceeding the `retentionSize` limit. In such cases, the `KubePersistentVolumeFillingUp` alert fires until the space on a PV is lower than the `retentionSize` limit.
+====
+
 .Prerequisites
 
 ifndef::openshift-dedicated,openshift-rosa[]