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 <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>