Add a comment about when it is possible to screenshare

Signed-off-by: Šimon Brandner <simon.bra.ag@gmail.com>

src/components/views/voip/CallView.tsx

@@ -505,6 +505,9 @@ export default class CallView extends React.Component<IProps, IState> {
         /> : null;
 
         let screensharingButton;
+        // Screensharing is possible, if we can send a second stream and identify
+        // it using SDPStreamMetadata or if we can replace the already existing
+        // usermedia track by a screensharing track
         if (this.props.call.opponentSupportsSDPStreamMetadata() || this.props.call.type === CallType.Video) {
             screensharingButton = (
                 <AccessibleButton