[Docs] Warn about searching across all fields wt. `query_string`
Warn about potential performance impact when a large number of fields
is used with query string query and no default field.
@@ -153,8 +153,11 @@ When not explicitly specifying the field to search on in the query
string syntax, the `index.query.default_field` will be used to derive
which field to search on. If the `index.query.default_field` is not specified,
the `query_string` will automatically attempt to determine the existing fields in the index's
-mapping that are queryable, and perform the search on those fields. Note that this will not
-include nested documents, use a nested query to search those documents.
+mapping that are queryable, and perform the search on those fields.
+This will not include nested documents, use a nested query to search those documents.
+
+NOTE: For mappings with a large number of fields, searching across all queryable
+fields in the mapping could be expensive.
[float]
==== Multi Field
