getsockopt.2: clarify that SO_TIMESTAMP is not 100% reliable

When SO_TIMESTAMP is set, the kernel will attempt to attach a timestamp as
ancillary data to each IP datagram that is received on the socket. However,
it may fail, for example due to insufficient memory. In that case the
packet will still be received but not timestamp will be attached.

Reviewed by: kib
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D21607