document the world module

src/world/mod.rs

@@ -1,4 +1,14 @@
-//! The game world. Regions, zones, and rooms.
+//! The game world.
+//!
+//! The world is divided up into zones and rooms. A zone can contain a
+//! collection of both rooms and zones. Each zone and room can specify if the
+//! players inside it are visible to the sibling rooms and zones.
+//!
+//! This allows for some granularity in the zone design. An example, there may
+//! be a zone for a city, containing a few rooms for gates and a few zones for
+//! different regions of the city. One might be the trade district, and each
+//! shop might be a single room, or zone with a few rooms depicting the layout
+//! of a large store.
 
 mod area;
 mod direction;