ptp: document struct ptp_clock_request members

It's arguable most people interested in configuring a PPS signal
want it as external output, not as kernel input. PTP_CLK_REQ_PPS
is for input though. Add documentation to nudge readers into
the correct direction.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Link: https://lore.kernel.org/r/20201117213826.18235-1-a.fatoum@pengutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>