added JAVA_HOME to the listed environment variables to improve #223.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@1723 71c3de6d-444a-0410-be80-ed276b4c234a

war/resources/env-vars.html

@@ -30,7 +30,8 @@
 
     <dt>JAVA_HOME</dt>
     <dd>If your job is configured to use a specific JDK, this variable is set to
-        the JAVA_HOME of the specified JDK.</dd>
+        the JAVA_HOME of the specified JDK. When this variable is set, <tt>PATH</tt>
+        is also updated to have <tt>$JAVA_HOME/bin</tt>.</dd>
 </dl>