Qt Multimedia: Add alt texts

qdoc: 'QtMultimedia': \image audiodevices.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image audiooutput-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image audiorecorder.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image audiosource-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image camera-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image qml-camera.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image qml-declarative-portrait.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image FlashControls.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image mediaplayerex.jpg is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image screencapture.jpg is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image qtmultimedia-examples-qml-media-player.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image qtmultimedia-examples-qml-media-player-settings.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image qmlvideo-overlay.jpg is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image qmlvideo-menu.jpg is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image video-qml-paint-rate.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image qmlrecorder.jpg is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image qml-recorder-control-bar-overview.gif is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image video-videographicsitem.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtMultimedia': \image video-videowidget.png is without a textual description, QDoc will not generate an alt text for the image.