Add an explanatory line of text before the list of FreeBSD version

values.  (I always forget where that actual file is -- even though
it is listed in the previous section, it makes the text flow a little
bit better to include it here, too.)

Approved by:	ceri (mentor)

en_US.ISO8859-1/books/porters-handbook/book.sgml

@@ -5053,6 +5053,10 @@ PLIST_SUB=      OCTAVE_VERSION=${OCTAVE_VERSION}</programlisting>
       <sect1 id="freebsd-versions">
 	<title>__FreeBSD_version values</title>
 
+	  <para>Here is a convenient list of
+	    <literal>__FreeBSD_version</literal> values as defined in
+	    <filename>sys/param.h</filename>:</para>
+
 		<table frame="none">
 		  <title>__FreeBSD_version values</title>
 		<tgroup cols="2">