Add warning about offline instrumentation.
diff --git a/org.jacoco.doc/docroot/doc/ant.html b/org.jacoco.doc/docroot/doc/ant.html
index 2eb8aa7..31c9584 100644
--- a/org.jacoco.doc/docroot/doc/ant.html
+++ b/org.jacoco.doc/docroot/doc/ant.html
@@ -707,6 +707,14 @@
<h2><a name="instrument">Task <code>instrument</code></a></h2>
+<p class="hint">
+ <b>Warning:</b> The preferred way for code coverage analysis with JaCoCo is
+ on-the-fly instrumentation. Offline instrumentation has several drawbacks and
+ should only be used if a specific scenario explicitly requires this mode.
+ Please consult <a href="offline.html">documentation</a> about offline
+ instrumentation before using this mode.
+</p>
+
<p>
This task is used for <a href="offline.html">offline instrumentation</a> of
class files. The task takes a set of files and writes instrumented