Require documentation by default for libstd

Adds documentation for various things that I understand.
Adds #[allow(missing_doc)] for lots of things that I don't understand.

src/libstd/task/mod.rs

@@ -33,6 +33,8 @@
  * ~~~
  */
 
+#[allow(missing_doc)];
+
 use prelude::*;
 
 use cast;