nfs(5): Replace the term "netid" in mount option descriptions

Mike Eisler noted that the use of the term "netid" in the descriptions
of the "proto=" option is not appropriate, since Linux does not allow
"udp6" or "tcp6".

Replaced the term "netid" with "transport" in nfs(5).

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Tom Talpey <Thomas.Talpey@netapp.com>
Signed-off-by: Steve Dickson <steved@redhat.com>