net: fix documentation of struct scm_timestamping

The scm_timestamping struct may return multiple non-zero fields, e.g.
when both software and hardware RX timestamping is enabled, or when the
SO_TIMESTAMP(NS) option is combined with SCM_TIMESTAMPING and a false
software timestamp is generated in the recvmsg() call in order to always
return a SCM_TIMESTAMP(NS) message.

CC: Richard Cochran <richardcochran@gmail.com>
CC: Willem de Bruijn <willemb@google.com>
Signed-off-by: NMiroslav Lichvar <mlichvar@redhat.com>
Acked-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/timestamping.txt

@@ -322,7 +322,7 @@ struct scm_timestamping {
 };
 
 The structure can return up to three timestamps. This is a legacy
-feature. Only one field is non-zero at any time. Most timestamps
+feature. At least one field is non-zero at any time. Most timestamps
 are passed in ts[0]. Hardware timestamps are passed in ts[2].
 
 ts[1] used to hold hardware timestamps converted to system time.
@@ -331,6 +331,12 @@ a HW PTP clock source, to allow time conversion in userspace and
 optionally synchronize system time with a userspace PTP stack such
 as linuxptp. For the PTP clock API, see Documentation/ptp/ptp.txt.
 
+Note that if the SO_TIMESTAMP or SO_TIMESTAMPNS option is enabled
+together with SO_TIMESTAMPING using SOF_TIMESTAMPING_SOFTWARE, a false
+software timestamp will be generated in the recvmsg() call and passed
+in ts[0] when a real software timestamp is missing. This happens also
+on hardware transmit timestamps.
+
 2.1.1 Transmit timestamps with MSG_ERRQUEUE
 
 For transmit timestamps the outgoing packet is looped back to the