Describe what _source.includes/excludes do (#38319)

docs/reference/search/request/source-filtering.asciidoc

@@ -54,7 +54,10 @@ GET /_search
 // CONSOLE
 
 Finally, for complete control, you can specify both `includes` and `excludes`
-patterns:
+patterns. If `includes` is not empty, then only fields that match one of the
+patterns in `includes` but none of the patterns in `excludes` are provided in
+`_source`. If `includes` is empty, then all fields are provided in `_source`,
+except for those that match a pattern in `excludes`.
 
 [source,js]
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