Fix best practice advice for freeing media element resources

This came up in the context of https://bugs.chromium.org/p/chromium/issues/detail?id=1144736#c72.

Per spec, removing the src attribute does not release resources, so the best practices section shouldn't suggest doing so. See https://html.spec.whatwg.org/multipage/media.html#location-of-the-media-resource.

source

@@ -39120,9 +39120,9 @@ dictionary <dfn dictionary>TrackEventInit</dfn> : <span>EventInit</span> {
   support three simultaneous videos. For this reason, it is a good practice to release resources
   held by <span data-x="media element">media elements</span> when they are done playing, either by
   being very careful about removing all references to the element and allowing it to be garbage
-  collected, or, even better, by removing the element's <code data-x="attr-media-src">src</code>
-  attribute and any <code>source</code> element descendants, and invoking the element's <code
-  data-x="dom-media-load">load()</code> method.</p>
+  collected, or, even better, by setting the element's <code data-x="attr-media-src">src</code>
+  attribute to an empty string. In cases where <code data-x="dom-media-srcObject">srcObject</code>
+  was set, instead set the <code data-x="dom-media-srcObject">srcObject</code> to null.</p>
 
   <p>Similarly, when the playback rate is not exactly 1.0, hardware, software, or format limitations
   can cause video frames to be dropped and audio to be choppy or muted.</p>