doc: async-hooks documentation

Author: Thorsten Lorenz <thlorenz@gmx.de>
Author: Andreas Madsen <amwebdk@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/13287


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>