doc: clarify Worker exit/message event ordering

Motivated by the fact that getting this wrong has led to flaky
tests in our test suite.

Refs: https://github.com/nodejs/node/pull/31637

PR-URL: https://github.com/nodejs/node/pull/31642


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>