Documentation for `thread_local!` should link to docs for `LocalKey`

The documentation for [`thread_local!`](http://doc.rust-lang.org/std/macro.thread_local!.html) isn't very usable right now. It should link to the docs for [`LocalKey`](http://doc.rust-lang.org/std/thread/struct.LocalKey.html). Or maybe some of the docs for `LocalKey` should be moved to `thread_local!`.

A similar situation applies to `scoped_thread_local!`.
