Rollup merge of #110049 - SkiFire13:localkey-with-docs-fix, r=workingjubilee

Don't claim `LocalKey::with` prevents a reference to be sent across threads

The documentation for `LocalKey` claims that `with` yields a reference that cannot be sent across threads, but this is false since you can easily do that with scoped threads. What it actually prevents is the reference from outliving the current thread.

Author: matthiaskrgr