GRF: Add extra road/tram type flag: towns cannot modify tiles

docs/newgrf-additions.html

@@ -238,6 +238,7 @@
 	<table>
 	<tr><th>Bit</th><th>Value</th><th>Meaning</th></tr>
 	<tr><td>0</td><td>1</td><td>Scripts (AI/GS) may not build this road/tram type</td></tr>
+	<tr><td>1</td><td>2</td><td>Towns may not modify tiles of this road/tram type in any way whatsoever</td></tr>
 	</table>
 	</p>
 	<p>This is indicated by the feature name: <font face="monospace">action0_roadtype_extra_flags</font>, version 1</p>