doc: Complete documentation for VideoStreams

Also quick clean up of the matching C++ files.

servers/audio/audio_stream.h

@@ -82,7 +82,7 @@ public:
 class AudioStream : public Resource {
 
 	GDCLASS(AudioStream, Resource);
-	OBJ_SAVE_TYPE(AudioStream) //children are all saved as AudioStream, so they can be exchanged
+	OBJ_SAVE_TYPE(AudioStream); // Saves derived classes with common type so they can be interchanged.
 
 protected:
 	static void _bind_methods();