Document that awareness attributes override custom preferences

When selecting replicas in a search, the coordinating node prefers nodes with
the same shard allocation awareness attributes. So even if a search specifies
the same custom preference values, different coordinating nodes may route it
differently because of their awareness attributes.

In 8.0, allocation awareness attributes no longer influence search replica
selection. So although this is a bug, we do not intend to fix it in 7.x or 6.x.
Instead, we document the behavior as a 'warning' and mention a system property
that can be used to disable the behavior.

Addresses #53054.

Author: jtibshirani

docs/reference/search/request/preference.asciidoc

@@ -115,6 +115,12 @@ and on a changing index this may mean that repeated searches may yield
 different results if they are executed on different shard copies which are in
 different refresh states.
 
+WARNING: If a node has <<allocation-awareness,shard allocation awareness>>
+attributes, then it will prefer shards with the same awareness attribute values.
+Because of these attributes, different coordinating nodes may route the same
+search differently, even if the searches use the same custom `preference` values.
+In recent {es} versions, awareness attributes no longer influence search routing.
+
 WARNING: The `_primary`, `_primary_first`, `_replica` and `_replica_first` are
 deprecated as their use is not recommended. They do not help to avoid
 inconsistent results that arise from the use of shards that have different