Docs: Add a warning about fields vs. source filtering.

Close #14470

docs/reference/search/request/fields.asciidoc

@@ -1,6 +1,11 @@
 [[search-request-fields]]
 === Fields
 
+WARNING: The `fields` parameter is about fields that are explicitly marked as
+stored in the mapping, which is off by default and generally not recommended.
+Use <<search-request-source-filtering,source filtering>> instead to select
+subsets of the original source document to be returned.
+
 Allows to selectively load specific stored fields for each document represented
 by a search hit.