avutil/stereo3d: consolidate group doxy

Make it a bit easier to grasp the grouping when not
unnecessarily splitting comment blocks.

Additionally do not try to add lavu_video_stereo3d to itself, resolving
the following doxy warning:
  warning: Refusing to add group lavu_video_stereo3d to itself

libavutil/stereo3d.h

@@ -31,19 +31,15 @@
 #include "frame.h"
 
 /**
- * @addtogroup lavu_video
- * @{
- *
  * @defgroup lavu_video_stereo3d Stereo3D types and functions
- * @{
- */
-
-/**
- * @addtogroup lavu_video_stereo3d
+ * @ingroup lavu_video
+ *
  * A stereoscopic video file consists in multiple views embedded in a single
  * frame, usually describing two views of a scene. This file describes all
  * possible codec-independent view arrangements.
- * */
+ *
+ * @{
+ */
 
 /**
  * List of possible 3D Types
@@ -226,7 +222,6 @@ const char *av_stereo3d_type_name(unsigned int type);
 int av_stereo3d_from_name(const char *name);
 
 /**
- * @}
  * @}
  */