Rollup merge of #128064 - ijackson:noop-waker-doc, r=workingjubilee

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.

Author: workingjubilee