Improve the local_data docs slightly

Remove references to local_data::Key and only mention the macro for how to
construct new keys into local data.

src/libstd/local_data.rs

@@ -17,9 +17,10 @@
 the TLS slot.  Useful for dynamic variables, singletons, and interfacing with
 foreign code with bad callback interfaces.
 
-To use, declare a static variable of the type you wish to store. The
-initialization should be `&local_data::Key`. This is then the key to what you
-wish to store.
+To declare a new key for storing local data of a particular type, use the
+`local_data_key!` macro. This macro will expand to a `static` item apppriately
+named and annotated. This name is then passed to the functions in this module to
+modify/read the slot specified by the key.
 
 ~~~{.rust}
 use std::local_data;
@@ -31,14 +32,14 @@
 local_data::get(key_int, |opt| assert_eq!(opt, Some(&3)));
 
 local_data::set(key_vector, ~[4]);
-local_data::get(key_int, |opt| assert_eq!(opt, Some(&~[4])));
+local_data::get(key_vector, |opt| assert_eq!(opt, Some(&~[4])));
 ~~~
 
-Casting 'Arcane Sight' reveals an overwhelming aura of Transmutation
-magic.
-
 */
 
+// Casting 'Arcane Sight' reveals an overwhelming aura of Transmutation
+// magic.
+
 use cast;
 use libc;
 use prelude::*;