doc: Mention MSG_ZEROCOPY implementation for UDP

MSG_ZEROCOPY implementation for UDP was merged in v5.0,
6e360f73 ("Merge branch 'udp-msg_zerocopy'").

Signed-off-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>