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

include/linux/ptp_clock_kernel.h

@@ -12,6 +12,19 @@
 #include <linux/pps_kernel.h>
 #include <linux/ptp_clock.h>
 
+/**
+ * struct ptp_clock_request - request PTP clock event
+ *
+ * @type:   The type of the request.
+ *	    EXTTS:  Configure external trigger timestamping
+ *	    PEROUT: Configure periodic output signal (e.g. PPS)
+ *	    PPS:    trigger internal PPS event for input
+ *	            into kernel PPS subsystem
+ * @extts:  describes configuration for external trigger timestamping.
+ *          This is only valid when event == PTP_CLK_REQ_EXTTS.
+ * @perout: describes configuration for periodic output.
+ *	    This is only valid when event == PTP_CLK_REQ_PEROUT.
+ */
 
 struct ptp_clock_request {
 	enum {