Improve docs for Waker::noop and LocalWaker::noop * Add a warning about a likely misuse. (See my commit message for longer rationale.) * Apply some probably-accidentally-omitted changes to `LocalWaker`'s docs * Add a comment about the clone-and-hack of the docs I have used [semantic linefeeds](https://rhodesmill.org/brandon/2012/one-sentence-per-line/) for the docs formatting.