doc: improve AsyncLocalStorage introduction

PR-URL: https://github.com/nodejs/node/pull/36946


Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>