NFS: Update help text for CONFIG_NFS_FS

Clean up: refresh the help text for Kconfig items related to the NFS
client.  Remove obsolete URLs, and make the language consistent among
the options.

Also move the ROOT_NFS config option next to the options related to the
NFS client.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>