Docs: Insist that setting size=0 will help performance.

docs/reference/search/request-body.asciidoc

@@ -59,7 +59,9 @@ And here is a sample response:
 
 `size`::
 
-    The number of hits to return. Defaults to `10`.
+    The number of hits to return. Defaults to `10`. If you do not care about
+    getting some hits back but only about the number of matches and/or
+    aggregations, setting the value to `0` will help performance.
 
 `search_type`::