Add Picture in Picture window to note on local playback devices

See w3c/picture-in-picture#191

index.html

@@ -339,7 +339,8 @@ <h3>
         </p>
         <div class="note">
           A <a>local playback device</a> might have extra outputs, like an
-          external display or speakers/headphones. As long as the switch of
+          external display or speakers/headphones, or a Picture-in-Picture
+          window [[Picture-in-Picture]]. As long as the switch of
           what output to use happens outside of the user agent on the
           system level, the playback is considered to happen on a <a>local
           playback device</a> for the purpose of this spec.