Fix MediaPlayer::playlist documentation.

Added missing '\since 5.6' tag.

Change-Id: I46e0ea8bad85fb1f51f5502766c13549c6dad9d5
Reviewed-by: Christian Stromme <christian.stromme@theqtcompany.com>

src/imports/multimedia/Video.qml

@@ -327,6 +327,8 @@ Item {
         This property holds the playlist used by the media player.
 
         Setting the \l playlist property resets the \l source to an empty string.
+
+        \since 5.6
     */
     property alias playlist:        player.playlist
 

src/imports/multimedia/qdeclarativeaudio.cpp

@@ -556,6 +556,8 @@ void QDeclarativeAudio::seek(int position)
     This property holds the playlist used by the media player.
 
     Setting the \l playlist property resets the \l source to an empty string.
+
+    \since 5.6
 */
 
 /*!
@@ -1186,6 +1188,8 @@ void QDeclarativeAudio::_q_mediaChanged(const QMediaContent &media)
     This property holds the playlist used by the media player.
 
     Setting the \l playlist property resets the \l source to an empty string.
+
+    \since 5.6
 */
 
 /*!