[Backport 7.x] Use filter_path for improving the search helpers performances (#1201)

docs/helpers.asciidoc

@@ -212,7 +212,8 @@ console.log(result)
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 === Search Helper
-A simple wrapper around the search API. Instead of returning the entire `result` object it will return only the search documents result.
+A simple wrapper around the search API. Instead of returning the entire `result` object it will return only the search documents source.
+For improving the performances, this helper automatically adds `filter_path=hits.hits._source` to the querystring.
 
 [source,js]
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@@ -281,6 +282,7 @@ for await (const result of scrollSearch) {
 === Scroll Documents Helper
 
 It works in the same way as the scroll search helper, but it returns only the documents instead. Note, every loop cycle will return you a single document, and you can't use the `clear` method.
+For improving the performances, this helper automatically adds `filter_path=hits.hits._source` to the querystring.
 
 [source,js]
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