Reduce chance of conflict with other agents (#555)

diff --git a/org.jacoco.doc/docroot/doc/implementation.html b/org.jacoco.doc/docroot/doc/implementation.html
index bea6f73..b1eda7f 100644
--- a/org.jacoco.doc/docroot/doc/implementation.html
+++ b/org.jacoco.doc/docroot/doc/implementation.html
@@ -215,7 +215,9 @@
 <p>
   The current JaCoCo Java agent implementation uses the 
   <code>ModifiedSystemClassRuntime</code> adding a field to the class
-  <code>java.util.UUID</code>.
+  <code>java.lang.UnknownError</code>. Versions 0.5.0 - 0.7.9 were adding field
+  to the class <code>java.util.UUID</code>, having bigger chance of conflict
+  with other agents.
 </p>