[media] videobuf2-core: Add it to device-drivers DocBook

Most of the stuff at videobuf2-core are ok for adding it to
DocBook.

Two notes here:
1) As videobuf2-core will be soon be changed, better to
   not spend too much efforts right now, as things will
   change soon;

2) struct vb2_queue has a number of private elements that are
   documented. As Kernel nano documentation format handles
   "private:" arguments, we need to put them on a separate
   comment block or to remove. Keeping the comments is
   obviously better ;)
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DocBook/device-drivers.tmpl

@@ -234,10 +234,10 @@ X!Isound/sound_firmware.c
 !Iinclude/media/v4l2-ctrls.h
 !Iinclude/media/v4l2-event.h
 !Iinclude/media/v4l2-dv-timings.h
+!Iinclude/media/videobuf2-core.h
 <!-- FIXME: Removed for now due to document generation inconsistency
 X!Iinclude/media/v4l2-mediabus.h
 X!Iinclude/media/videobuf2-memops.h
-X!Iinclude/media/videobuf2-core.h
 -->
 
   </chapter>

include/media/videobuf2-core.h

@@ -362,7 +362,9 @@ struct v4l2_fh;
  *		start_streaming() can be called. Used when a DMA engine
  *		cannot be started unless at least this number of buffers
  *		have been queued into the driver.
- *
+ */
+/*
+ * Private elements (won't appear at the DocBook):
  * @mmap_lock:	private mutex used when buffers are allocated/freed/mmapped
  * @memory:	current memory type used
  * @bufs:	videobuf buffer structures
@@ -405,7 +407,7 @@ struct vb2_queue {
 	gfp_t				gfp_flags;
 	u32				min_buffers_needed;
 
-/* private: internal use only */
+	/* private: internal use only */
 	struct mutex			mmap_lock;
 	enum v4l2_memory		memory;
 	struct vb2_buffer		*bufs[VIDEO_MAX_FRAME];
@@ -482,7 +484,8 @@ size_t vb2_read(struct vb2_queue *q, char __user *data, size_t count,
 		loff_t *ppos, int nonblock);
 size_t vb2_write(struct vb2_queue *q, const char __user *data, size_t count,
 		loff_t *ppos, int nonblock);
-/**
+
+/*
  * vb2_thread_fnc - callback function for use with vb2_thread
  *
  * This is called whenever a buffer is dequeued in the thread.
@@ -575,7 +578,6 @@ static inline void vb2_set_plane_payload(struct vb2_buffer *vb,
  * vb2_get_plane_payload() - get bytesused for the plane plane_no
  * @vb:		buffer for which plane payload should be set
  * @plane_no:	plane number for which payload should be set
- * @size:	payload in bytes
  */
 static inline unsigned long vb2_get_plane_payload(struct vb2_buffer *vb,
 				 unsigned int plane_no)