[media] v4l: Document timestamp behaviour to correspond to reality

Document that monotonic timestamps are taken after the corresponding frame
has been received, not when the reception has begun. This corresponds to the
reality of current drivers: the timestamp is naturally taken when the
hardware triggers an interrupt to tell the driver to handle the received
frame.

Remove the note on timestamp accuracy as it is fairly subjective what is
actually an unstable timestamp.

Also remove explanation that output buffer timestamps can be used to delay
outputting a frame.

Remove the footnote saying we always use realtime clock.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>