javadocs: explain a little more about draw bridges

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@570 d9718cc8-9f43-0410-858b-315f434eb58c

src/micropolisj/engine/MapScanner.java

@@ -275,6 +275,14 @@ class MapScanner
 
 	/**
 	 * Called when the current tile is a road bridge over water.
+	 * Handles the draw bridge. For the draw bridge to appear,
+	 * there must be a boat on the water, the boat must be
+	 * within a certain distance of the bridge, it must be where
+	 * the map generator placed 'channel' tiles (these are tiles
+	 * that look just like regular river tiles but have a different
+	 * numeric value), and you must be a little lucky.
+	 *
+	 * @return true if the draw bridge is open; false otherwise
 	 */
 	boolean doBridge()
 	{