doc: simplify async_hooks overview

Explain the purpose of async_hooks simply in the overview.

PR-URL: https://github.com/nodejs/node/pull/31660


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>