doc: add notes to http.get options

Extra notes that options doesn't include the prototype when copied

Fixes: nodejs#12092
PR-URL: nodejs#12124
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/http.md

@@ -1544,6 +1544,7 @@ added: v0.3.6
 
 * `options` {Object | string} Accepts the same `options` as
   [`http.request()`][], with the `method` always set to `GET`.
+  Properties that are inherited from the prototype are ignored.
 * `callback` {Function}
 * Returns: {http.ClientRequest}