Clarify (lack of) backup redistribution on data structure level [HZG-…

…156]

docs/modules/fault-tolerance/pages/backups.adoc

@@ -7,6 +7,17 @@ The distribution happens on the partition level; the data and its backups are st
 memory partitions. See the xref:overview:data-partitioning.adoc[Data Partitioning] and xref:clusters:partition-group-configuration.adoc[Partition Grouping]
 for more information about the partitioning.
 
+[IMPORTANT]
+====
+Hazelcast does not support backup redistribution on data structure level. For example, for a map with a nonzero backup count,
+
+1. increasing the backup count does **not** create additional copies of existing map entries,
+2. decreasing the backup count does **not** remove any backups of existing map entries, so some backups will have stale data if the map entries are overwritten, and
+3. if some backups are lost due to node failure, additional copies are **not** created on other nodes to meet the backup count.
+
+Thus, it is unsupported to change the backup count of a data structure once it was populated with data.
+====
+
 When a member in your cluster is lost, Hazelcast redistributes the backups
 on the remaining members so that every partition has a backup.
 The number of backups is configurable.