mount.nfs mapage: clear up confusion between 'proto' and 'transport'

The mount option "proto=" actually set the "transport" which in
netconfig usage is the pairing of a protocol (e.g. UDP, TCP) with
a protocol family (e.g. INET, INET6).

This can cause confusion if people naively except "proto=udp" to work
equally well on IPv6.

So add some text to both nfs(5) and nfsmount.conf(5) to hopefully
clarify this.

Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>