diff --git a/CHANGES.txt b/CHANGES.txt
index a63bd1260f..22893348dc 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,4 +1,5 @@
 3.0.12
+ * Update c.yaml doc for offheap memtables (CASSANDRA-13179)
  * Faster StreamingHistogram (CASSANDRA-13038)
  * Legacy deserializer can create unexpected boundary range tombstones (CASSANDRA-13237)
  * Remove unnecessary assertion from AntiCompactionTest (CASSANDRA-13070)
diff --git a/conf/cassandra.yaml b/conf/cassandra.yaml
index 790dfd743b..61d38445de 100644
--- a/conf/cassandra.yaml
+++ b/conf/cassandra.yaml
@@ -397,7 +397,11 @@ concurrent_materialized_view_writes: 32
 # Specify the way Cassandra allocates and manages memtable memory.
 # Options are:
 #   heap_buffers:    on heap nio buffers
-#   offheap_buffers: off heap (direct) nio buffers
+#
+# Note: offheap_buffers are not supported in Cassandra 3.0 - 3.3.
+# They have been re-introduced in Cassandra 3.4. For details see
+# https://issues.apache.org/jira/browse/CASSANDRA-9472 and
+# https://issues.apache.org/jira/browse/CASSANDRA-11039
 memtable_allocation_type: heap_buffers
 
 # Total space to use for commit logs on disk.