ESQL: Mark union types as experimental (#111297)

docs/reference/esql/esql-multi-index.asciidoc

@@ -103,7 +103,7 @@ FROM events_*
 
 [source,bash]
 ----
-Cannot use field [client_ip] due to ambiguities being mapped as 
+Cannot use field [client_ip] due to ambiguities being mapped as
 [2] incompatible types:
     [ip] in [events_ip],
     [keyword] in [events_keyword]
@@ -113,12 +113,14 @@ Cannot use field [client_ip] due to ambiguities being mapped as
 [[esql-multi-index-union-types]]
 === Union types
 
+experimental::[]
+
 {esql} has a way to handle <<esql-multi-index-invalid-mapping, field type mismatches>>. When the same field is mapped to multiple types in multiple indices,
 the type of the field is understood to be a _union_ of the various types in the index mappings.
 As seen in the preceding examples, this _union type_ cannot be used in the results,
 and cannot be referred to by the query
 -- except when it's passed to a type conversion function that accepts all the types in the _union_ and converts the field
-to a single type. {esql} offers a suite of <<esql-type-conversion-functions,type conversion functions>> to achieve this. 
+to a single type. {esql} offers a suite of <<esql-type-conversion-functions,type conversion functions>> to achieve this.
 
 In the above examples, the query can use a command like `EVAL client_ip = TO_IP(client_ip)` to resolve
 the union of `ip` and `keyword` to just `ip`.