added missing javadoc

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/digester/trunk@1197155 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/src/main/java/org/apache/commons/digester3/binder/DigesterLoader.java b/src/main/java/org/apache/commons/digester3/binder/DigesterLoader.java
index c99d8c2..ad070da 100644
--- a/src/main/java/org/apache/commons/digester3/binder/DigesterLoader.java
+++ b/src/main/java/org/apache/commons/digester3/binder/DigesterLoader.java
@@ -115,6 +115,10 @@
      */
     private ClassLoader classLoader;
 
+    /**
+     * An optional class that substitutes values in attributes and body text. This may be null and so a null check is
+     * always required before use.
+     */
     private Substitutor substitutor;
 
     /**