Add help text for name_scan filter options.
Add help text for name_scan filter options.

Document the filter options possible for name_scan.  They have been added
in https://github.com/namecoin/namecoin-core/pull/269, but that missed
adding them to the help.

pataquets/namecoin-core,namecoin/namecoin-core,domob1812/namecore,pataquets/namecoin-core,namecoin/namecore,namecoin/namecoin-core,domob1812/namecore,domob1812/namecore,namecoin/namecore,namecoin/namecoin-core,namecoin/namecore,domob1812/namecore,pataquets/namecoin-core,namecoin/namecore,namecoin/namecoin-core,namecoin/namecore,domob1812/namecore,namecoin/namecore,namecoin/namecoin-core,pataquets/namecoin-core,pataquets/namecoin-core,domob1812/namecore,namecoin/namecoin-core,pataquets/namecoin-core