Add notes about required kernel options for PF as a module and explain how

to build an IPv6-less setup.

Approved by:	simon
Inspired by PR:	kern/70401

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

@@ -299,6 +299,14 @@
 	rc.conf statement <literal>pf_enable="YES"</literal> is used.
 	The loadable module was created with &man.pflog.4; logging
 	enabled.</para>
+
+      <note>
+	<para>The module assumes the presence of <literal>options
+	    INET</literal> and <literal>device bpf</literal>.  Unless
+	  <literal>NOINET6</literal> (for example in &man.make.conf.5;)
+	  was defined during the build, it also requires <literal>options
+	    INET6</literal>.</para>
+      </note>
     </sect2>
 
     <sect2>