media: videobuf2-core.h: improve buf_struct_size documentation

Explicitly document that the driver-specific buffer structure
must start with the subsystem-specific struct (vb2_v4l2_buffer
in the case of V4L2).

Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

include/media/videobuf2-core.h

···
  *		by the vb2 core.
  * @buf_struct_size: size of the driver-specific buffer structure;
  *		"0" indicates the driver doesn't want to use a custom buffer
- *		structure type. for example, ``sizeof(struct vb2_v4l2_buffer)``
- *		will be used for v4l2.
+ *		structure type. In that case a subsystem-specific struct
+ *		will be used (in the case of V4L2 that is
+ *		``sizeof(struct vb2_v4l2_buffer)``). The first field of the
+ *		driver-specific buffer structure must be the subsystem-specific
+ *		struct (vb2_v4l2_buffer in the case of V4L2).
  * @timestamp_flags: Timestamp flags; ``V4L2_BUF_FLAG_TIMESTAMP_*`` and
  *		``V4L2_BUF_FLAG_TSTAMP_SRC_*``
  * @gfp_flags:	additional gfp flags used when allocating the buffers.