async_hooks,doc: some async_hooks improvements

Update docs and type checking for AsyncResource type

PR-URL: nodejs/node#15103
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: Andreas Madsen <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Trevor Norris <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>