Add GRF variable to use extra station names even when default names remain

docs/newgrf-additions-nml.html

@@ -271,6 +271,12 @@ item (FEAT_GLOBALVARS) {
 			</dl>
 		</td>
 	</tr>
+	<tr><td>extra_station_names_probability</td><td>0 .. 255</td>
+		<td>
+			Sets the probability that an extra station name is used even when the available default names have not been exhausted.<br />
+			Some station names are always used first even when this is non-zero.
+		</td>
+	</tr>
 	</table>
 	<p>Syntax example:
 		<pre class="code">