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

TI-RPC introduced the concept of "netid" which is a string that is
mapped to a set of transport capabilities via a netconfig database.
RPC services register a netid and bindaddr with their local rpcbind
daemon to advertise their ability to support particular transports.

Mike Eisler noted that the use of the term "netid" in nfs(5) is not
appropriate, since Linux does not treat the value of the proto= or
mountproto= options as a netid proper, but rather to select a
particular transport capability provided locally on the client.

The Linux NFS client currently uses a simple internal mapping between
these names and its own transport capabilities rather than using the
names as part of an rpcbind query, thus these strings are really not
netids.  They are more akin to what TI-RPC calls "protocol names".

Remove the term "netid" from nfs(5) for now.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Mike Eisler <mike.eisler@netapp.com>
Signed-off-by: Steve Dickson <steved@redhat.com>