Change: rename setting "max_heightlevel" to "map_height_limit"

This better reflects what it is, and hopefully removes a bit of
the confusion people are having what this setting actually does.

Additionally, update the text on the setting to better inform
users what it is doing exactly, so they can make an educated
decision on how to change it.

Next commit will introduce an "auto" value, which should be the
new default. The rename has as added benefit that everyone will
start out on the "auto" value.

src/tgp.cpp

@@ -236,7 +236,8 @@ static height_t TGPGetMaxHeight()
 
 	int map_size_bucket = std::min(MapLogX(), MapLogY()) - MIN_MAP_SIZE_BITS;
 	int max_height_from_table = max_height[_settings_game.difficulty.terrain_type][map_size_bucket];
-	return I2H(std::min<uint>(max_height_from_table, _settings_game.construction.max_heightlevel));
+
+	return I2H(std::min<uint>(max_height_from_table, _settings_game.construction.map_height_limit));
 }
 
 /**