NFS: update documentation for the nfs4_unique_id parameter

The documentation for nfs4_unique_id is out-of-date.  In particular it
claim that when nfs4_unique_id is set, the host name is not used.  since
Commit 55b59293 ("NFSv4: Fix nfs4_init_uniform_client_string for net
namespaces") both the unique_id AND the host name are used.

Update the documentation to match the code.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>