async_hooks,doc: some async_hooks improvements

Update docs and type checking for AsyncResource type

PR-URL: https://github.com/nodejs/node/pull/15103


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>