Show how to set resolution and frame rate (#2698)

* Show how to set resolution and frame rate

I was searching myself crazy on how to set the resolution and frame rate, and even thought that it's impossible to control,
until I found that they are set in getUserMedia().
Make a note where this is set, to save others the headache.

* Make it a second note box

* Add notecard class

files/en-us/web/api/mediarecorder/mediarecorder/index.html

@@ -66,6 +66,11 @@ <h3 id="Parameters">Parameters</h3>
         adopted for video is 2.5Mbps, while the audio default is adaptive, depending upon
         the sample rate and the number of channels.</p>
     </div>
+    <div class="notecard note">
+      <p>Video resolution, frame rate and similar settings are specified as constraints
+        when calling {{domxref("MediaDevices.getUserMedia", "getUserMedia()")}},
+        not here in the MediaRecorder API.</p>
+    </div>
   </dd>
 </dl>