Better message in case doxygen -u is used

In case of obsolete or not configured entries in the Doxyfile and the option -u is used a message like:
<pre>
  Warning: Tag `XML_DTD' at line 1821 of file Doxyfile has become obsolete.
  To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
</pre>
appears, but the item is removed from the Doxyfile.
This patch updates the message (and improves layout) to:
<pre>
  Warning: Tag `XML_DTD' at line 1821 of file \`Doxyfile' has become obsolete.
           This tag has been removed.
</pre>