add a comment in the docs about env key change

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@446984 13f79535-47bb-0310-9956-ffa450edef68

docs/manual/CoreTasks/exec.html

@@ -290,7 +290,12 @@ system command via nested <code>&lt;env&gt;</code> elements.</p>
   </tr>
   <tr>
     <td valign="top">key</td>
-    <td valign="top">The name of the environment variable.</td>
+    <td valign="top">
+      The name of the environment variable.
+      <br/>
+      <em>Note: (Since Ant 1.7)</em>
+      For windows, the name is case-insensitive.
+    </td>
     <td align="center" valign="top">Yes</td>
   </tr>
   <tr>