Some cleanup in std::io::buffered

`Vec` is now used for the internal buffer instead of `~[]`. Some module
level documentation somehow ended up attached to `BufferedReader` so I
fixed that as well.
auto merge of #13092 : sfackler/rust/buffer-vec, r=thestinger

`Vec` is now used for the internal buffer instead of `~[]`. Some module
level documentation somehow ended up attached to `BufferedReader` so I
fixed that as well.