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/path.rs

@@ -14,6 +14,8 @@ Cross-platform file path handling
 
 */
 
+#[allow(missing_doc)];
+
 use container::Container;
 use cmp::Eq;
 use libc;