Update docs.html

See #597

docs/docs.html

@@ -415,7 +415,8 @@ var csv = Papa.unparse({
 									<code>transformHeader</code>
 								</td>
 								<td>
-									A function to apply on each header. Requires <code>header</code> to be <code>true</code>. The function receives the header as its first argument.
+									A function to apply on each header. Requires <code>header</code> to be <code>true</code>. The function receives the header as its first argument.<br>
+									Only available starting with version 5.0.
 								</td>
 							</tr>
 							<tr>