Add some explanation

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@10456 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

chapter05/util-linux.xml

@@ -73,7 +73,11 @@
         <term><envar>PKG_CONFIG=""</envar></term>
         <listitem>
           <para>Setting this envronment variable prevents adding unneeded
-          features that may be available on the host.</para>
+          features that may be available on the host. Note that the location
+          shown for setting this environment variable is different from other
+          LFS sections where variables are set preceeding the command.  This
+          location is shown to demonstrate an alternative way of setting an
+          environment variable when using configure.</para>
         </listitem>
       </varlistentry>
      </variablelist>