Improve error message displayed when processing instrumented classes (#703)

diff --git a/org.jacoco.doc/docroot/doc/changes.html b/org.jacoco.doc/docroot/doc/changes.html
index ce2475c..5299b4e 100644
--- a/org.jacoco.doc/docroot/doc/changes.html
+++ b/org.jacoco.doc/docroot/doc/changes.html
@@ -38,6 +38,13 @@
       (GitHub <a href="https://github.com/jacoco/jacoco/issues/672">#672</a>).</li>
 </ul>
 
+<h3>Non-functional Changes</h3>
+<ul>
+  <li>Improved error message when already instrumented classes are used for
+      instrumentation or analysis
+      (GitHub <a href="https://github.com/jacoco/jacoco/issues/703">#703</a>).</li>
+</ul>
+
 <h2>Release 0.8.1 (2018/03/21)</h2>
 
 <h3>New Features</h3>