hashmap: improve docstring

Author: thestinger

src/libcore/hashmap.rs

@@ -8,7 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! Sendable hash maps.
+//! An unordered map and set type implemented as hash tables
+//!
+//! The tables use a keyed hash with new random keys generated for each container, so the ordering
+//! of a set of keys in a hash table is randomized.
 
 /// Open addressing with linear probing.
 pub mod linear {