Add note about voting-only in default distrib

Author: ywelsch

docs/reference/cluster.asciidoc

@@ -48,6 +48,9 @@ means that filters such as `master:false` which remove nodes from the chosen
 subset are only useful if they come after some other filters. When used on its
 own, `master:false` selects no nodes.
 
+NOTE: Designating nodes as `voting_only` and using `voting_only` in node
+filters requires the default distribution of Elasticsearch.
+
 Here are some examples of the use of node filters with the
 <<cluster-nodes-info,Nodes Info>> APIs.