Make QAudioSystemPlugin doc visible.

Change-Id: I0384fd6069349f321e60ffedd03834f1c46f634b
Reviewed-by: Yoann Lopes <yoann.lopes@qt.io>

src/multimedia/audio/qaudiosystem.cpp

@@ -48,7 +48,6 @@ QT_BEGIN_NAMESPACE
     \ingroup multimedia
     \ingroup multimedia_audio
     \inmodule QtMultimedia
-    \internal
 
     This class implements the audio functionality for
     QAudioDeviceInfo, i.e., QAudioDeviceInfo keeps a
@@ -112,7 +111,6 @@ QT_BEGIN_NAMESPACE
 
     \ingroup multimedia
     \inmodule QtMultimedia
-    \internal
 
     QAbstractAudioOutput implements audio functionality for
     QAudioOutput, i.e., QAudioOutput routes function calls to
@@ -252,7 +250,6 @@ QT_BEGIN_NAMESPACE
 
     \ingroup multimedia
     \inmodule QtMultimedia
-    \internal
 
     QAudioDeviceInput keeps an instance of QAbstractAudioInput and
     routes calls to functions of the same name to QAbstractAudioInput.

src/multimedia/audio/qaudiosystemplugin.cpp

@@ -58,7 +58,6 @@ QAudioSystemPluginExtension::~QAudioSystemPluginExtension()
     \ingroup multimedia
     \ingroup multimedia_audio
     \inmodule QtMultimedia
-    \internal
 
     Writing a audio plugin is achieved by subclassing this base class,
     reimplementing the pure virtual functions availableDevices(),