Docs: clarification of allocation awareness w.r.t. rack failures

Closes #11908

docs/reference/modules/cluster/allocation_awareness.asciidoc

@@ -44,7 +44,9 @@ nodes.
 
 Now, if we start two more nodes with `node.rack_id` set to `rack_two`,
 Elasticsearch will move shards across to the new nodes, ensuring (if possible)
-that the primary and replica shards are never in the same rack.
+that no two copies of the same shard will be in the same rack. However if `rack_two`
+were to fail, taking down both of its nodes, Elasticsearch will still allocate the lost
+shard copies to nodes in `rack_one`. 
 
 .Prefer local shards
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