[master] mention class and its default

src/bin/xfrin/b10-xfrin.xml

@@ -103,7 +103,7 @@ in separate zonemgr process.
       <command>b10-xfrin</command> daemon.
       The list items are:
       <varname>name</varname> (the zone name),
-<!-- TODO: class string -->
+      <varname>class</varname> (defaults to <quote>IN</quote>),
       <varname>master_addr</varname> (the zone master to transfer from),
       <varname>master_port</varname> (defaults to 53), and
       <varname>tsig_key</varname> (optional TSIG key to use).