Document "firewall_type" in this chapter.

PR:	38772

en_US.ISO8859-1/books/handbook/firewalls/chapter.sgml

@@ -2231,7 +2231,15 @@ options    IPV6FIREWALL_DEFAULT_TO_ACCEPT</programlisting>
 
       <programlisting>firewall_enable="YES"</programlisting>
 
-      <para>Set the script to run to activate your rules:</para>
+      <para>To select one of the default firewall types provided by
+	&os;, select one by reading the
+	<filename>/etc/rc.firewall</filename> file and place it in
+	the following:</para>
+
+      <programlisting>firewall_type="open"</programlisting>
+
+      <para>Or load custom rules by setting the following variable to the
+	file containing them:</para>
 
       <programlisting>firewall_script="/etc/ipfw.rules"</programlisting>