Update manual entry for javac to document when tempdir is used

PR: 26559
Obtained from: Steven Scheffler


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@276006 13f79535-47bb-0310-9956-ffa450edef68

docs/manual/CoreTasks/javac.html

@@ -341,6 +341,8 @@ invoking the compiler.</p>
   <tr>
     <td valign="top">tempdir</td>
     <td valign="top">Where Ant should place temporary files.
+      This is only used if the task is forked and the
+      command line args length exceeds 4k.
       <em>Since Ant 1.6</em>.</td>
     <td align="center" valign="top">No; default is the current working
        directory.</td>