V4L/DVB: V4L: File handles: Add documentation

Add documentation on using V4L2 file handles (v4l2_fh) in V4L2 drivers.

Signed-off-by: Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>