doc,worker: revise worker_threads.md

Rewrite worker_threads introductory material focusing on simpler and
shorter sentences.

PR-URL: https://github.com/nodejs/node/pull/25402


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>