nfsd: rename nfsd_serv_ prefixed methods and variables with nfsd_net_

Also update Documentation/filesystems/nfs/localio.rst accordingly
and reduce the technical documentation debt that was previously
captured in that document.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>