improve documentation of the regexp mapper.

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@270086 13f79535-47bb-0310-9956-ffa450edef68

docs/manual/CoreTypes/mapper.html

@@ -243,6 +243,14 @@ href="http://www.cacas.org/~wes/java/" target="_top">gnu.regexp</a> or <a
 href="http://www.crocodile.org/~sts/Rex/" target="_top">gnu.rex</a> with Ant, see <a
 href="http://marc.theaimsgroup.com/?l=ant-dev&m=97550753813481&w=2" target="_top">this</a>
 article.</p>
+<p>This means, you need <code>optional.jar</code> from the Ant release
+you are using <strong>and</strong> one of the supported regular
+expression libraries.  Make sure, both will be loaded from the same
+classpath, that is either put them into your <code>CLASSPATH</code>,
+<code>ANT_HOME/lib</code> directory or a nested
+<code>&lt;classpath&gt;</code> element of the mapper - you cannot have
+<code>optional.jar</code> in <code>ANT_HOME/lib</code> and the library
+in a nested <code>&lt;classpath&gt;</code>.</p>
 <p>Ant will choose the regular-expression library based on the
 following algorithm:</p>
 <ul>