Update doc for extended attributes

[HereThereBeDragons]

change xattr to table and add documentation about multipage attr, add example to restricting xattr access

issue #2922
pr #3355

documentation of the following might be missing (waiting for feedback if they should be added):

  Register("user.hitrate", new HitrateMagicXattr());
  Register("user.proxy_list", new ProxyListMagicXattr());
  Register("user.proxy_list_external", new ProxyListExternalMagicXattr());
  Register("user.timestamp_last_ioerr", new TimestampLastIOErrMagicXattr());
  Register("user.useddirp", new UsedDirPMagicXattr());
  Register("user.direct_io", new DirectIoMagicXattr());

  Register("user.authz", new AuthzMagicXattr());
  Register("user.external_url", new ExternalURLMagicXattr());

[jblomer]

Very nice!