Add NewGRF global flag to allow rocky tiles in tropic desert zones

docs/newgrf-additions-nml.html

@@ -323,6 +323,11 @@ item (FEAT_GLOBALVARS) {
 			Some station names are always used first even when this is non-zero.
 		</td>
 	</tr>
+	<tr><td>allow_rocks_in_desert</td><td>0 or 1</td>
+		<td>
+			Sets whether rocky tiles are allowed to generate in and remain in desert zones (tropical climate).
+		</td>
+	</tr>
 	</table>
 	<p>Syntax example:
 		<pre class="code">