net: fix documentation of struct scm_timestamping

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 <[email protected]>
CC: Willem de Bruijn <[email protected]>
Signed-off-by: Miroslav Lichvar <[email protected]>
Acked-by: Willem de Bruijn <[email protected]>
Signed-off-by: David S. Miller <[email protected]>