Add missing allow_missing_docs

Author: GuillaumeGomez

src/libstd/sys/windows/ext/mod.rs

@@ -18,6 +18,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 #![doc(cfg(windows))]
+#![allow(missing_docs)]
 
 pub mod ffi;
 pub mod fs;