io-safety: add missing link reference definitions

This RFC contained some links that referred to Rust symbols but lacked a
Markdown link reference definition, so they were just parsed as normal
text.

Test Plan:
In the rendered document, the character `[` now only appears as part of
the phrase `&[u8]`.

wchargin-branch: io-safety-link-refs
wchargin-source: 473004fb54129179a3b6df3e588365a998e8b131

Author: wchargin

text/3128-io-safety.md

@@ -429,10 +429,12 @@ patiently answering my many questions!
 [`RawFd`]: https://doc.rust-lang.org/stable/std/os/unix/io/type.RawFd.html
 [`RawHandle`]: https://doc.rust-lang.org/stable/std/os/windows/io/type.RawHandle.html
 [`RawSocket`]: https://doc.rust-lang.org/stable/std/os/windows/io/type.RawSocket.html
+[`AsRawFd::as_raw_fd`]: https://doc.rust-lang.org/stable/std/os/unix/io/trait.AsRawFd.html#tymethod.as_raw_fd
 [`FromRawFd::from_raw_fd`]: https://doc.rust-lang.org/stable/std/os/unix/io/trait.FromRawFd.html#tymethod.from_raw_fd
 [`from_raw_fd`]: https://doc.rust-lang.org/stable/std/os/unix/io/trait.FromRawFd.html#tymethod.from_raw_fd
 [`from_raw_handle`]: https://doc.rust-lang.org/stable/std/os/windows/io/trait.FromRawHandle.html#tymethod.from_raw_handle
 [`from_raw_socket`]: https://doc.rust-lang.org/stable/std/os/windows/io/trait.FromRawSocket.html#tymethod.from_raw_socket
+[`std::mem::forget`]: https://doc.rust-lang.org/std/mem/fn.forget.html
 [`SockRef::from`]: https://docs.rs/socket2/0.4.0/socket2/struct.SockRef.html#method.from
 [`unsafe_io::OwnsRaw`]: https://docs.rs/unsafe-io/0.6.2/unsafe_io/trait.OwnsRaw.html
 [LLVM's pointer aliasing rules]: http://llvm.org/docs/LangRef.html#pointer-aliasing-rules