Add prod warning to Composite Aggregation

[7.14 update](http://github.com/elastic/elasticsearch/pull/74559) exacerbated performance concerns on composite aggregations & [fix in discussion](#78313) is either reverting or will come in 7.16. In the mean time users need disclaimer that before change this search was expensive & esp. now or w/upgrade may impact cluster performance.

docs/reference/aggregations/bucket/composite-aggregation.asciidoc

@@ -4,6 +4,10 @@
 <titleabbrev>Composite</titleabbrev>
 ++++
 
+[WARNING]
+While this query _can_ be used in Production, it is more expensive than a normal
+terms aggregation and should be load tested for a use case before go-live.
+
 A multi-bucket aggregation that creates composite buckets from different sources.
 
 Unlike the other `multi-bucket` aggregations, you can use the `composite`