std: render the vec_ng docs.

These are wildly incomplete, but having something there is better than
nothing, e.g. so that people know it exists, and many of the functions
behaviour can be guessed from the name or by checking the source: it's
knowing they exist at all that's the hard part.

Author: huonw

src/libstd/vec_ng.rs

@@ -8,8 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// Migrate documentation over from `std::vec` when it is removed.
-#[doc(hidden)];
+// Migrate documentation over from `std::vec` progressively.  (This is
+// shown in docs so that people have something to refer too, even if
+// the page is rather empty.)
+#[allow(missing_doc)];
 
 use cast::{forget, transmute};
 use clone::Clone;