Document that `RawWakerVTable` functions must be thread-safe. Also add some intra-doc links and more high-level explanation of how `Waker` is used, while I'm here. Context: https://internals.rust-lang.org/t/thread-safety-of-rawwakervtables/17126
Document that `RawWakerVTable` functions must be thread-safe. Also add some intra-doc links and more high-level explanation of how `Waker` is used, while I'm here. Context: https://internals.rust-lang.org/t/thread-safety-of-rawwakervtables/17126