Added documentation for the new prefix attribute

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@271527 13f79535-47bb-0310-9956-ffa450edef68

docs/manual/CoreTasks/property.html

@@ -108,6 +108,12 @@ href="../using.html#built-in-props">here</a>.</p>
       elsewhere..</td>
     <td align="center" valign="top">No</td>
   </tr>
+  <tr>
+    <td valign="top">prefix</td> 
+    <td valign="top">Prefix to apply to properties loaded using <code>file</code>
+    or <code>resource</code>. A "." is appended to the prefix if not specified.</td>
+    <td align="center" valign="top">No</td>
+  </tr>
 </table>
 <h3>Parameters specified as nested elements</h3>
 <h4>classpath</h4>