Document touch() doesn't clear the stat cache.

reference/filesystem/functions/touch.xml

@@ -135,6 +135,27 @@ if (!touch('some_file.txt', $time)) {
  <refsect1 role="notes">
   &reftitle.notes;
   &note.filesystem-time-res;
+  <note>
+   <simpara>
+    Updates to the <literal>mtime</literal> or <literal>atime</literal>
+    of a file will not be reflected immediately in filesystem operations
+    such as <function>filemtime</function>.  That's because the information
+    is cached, and this function does not automatically clear the cache.
+    To have changes reflected in the application immediately, call
+    <function>clearstatcache</function> on the file.
+   </simpara>
+  </note>
+ </refsect1>
+
+ <refsect1 role="seealso">
+  &reftitle.seealso;
+  <para>
+   <simplelist>
+    <member><function>clearstatcache</function></member>
+    <member><function>fileatime</function></member>
+    <member><function>filemtime</function></member>
+   </simplelist>
+  </para>
  </refsect1>
 
 </refentry>