Documentation tweaks.

Mostly move widgets classes into the QtMultimediaWidgets module, but
also mark a few mostly internal playlist classes as \internal.  A few other
tweaks.

Change-Id: If0e376410bf2187cafb24f583108c6beb7b910a9
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>

src/multimedia/playback/qmediaplaylistioplugin.cpp

@@ -45,6 +45,7 @@ QT_BEGIN_NAMESPACE
 
 /*!
     \class QMediaPlaylistReader
+    \internal
 
     \brief The QMediaPlaylistReader class provides an interface for reading a playlist file.
     \inmodule QtMultimedia
@@ -86,6 +87,7 @@ QMediaPlaylistReader::~QMediaPlaylistReader()
 
 /*!
     \class QMediaPlaylistWriter
+    \internal
 
     \brief The QMediaPlaylistWriter class provides an interface for writing a playlist file.