add some module docstrings

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 {

src/libcore/ops.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// Core operators
+//! Traits for the built-in operators
 
 #[lang="drop"]
 pub trait Drop {

src/libstd/base64.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+//! Base64 binary-to-text encoding
+
 use core::iter;
 use core::str;
 use core::vec;