doc: expand on promises and async_hooks

AsyncHooks have a few subtleties with being able to track promises. This commit adds a section to the docs that explains things the issues.

Fixes: https://github.com/nodejs/node/issues/18520

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc:async_hooks

/cc @nodejs/async_hooks @nodejs/diagnostics EDIT: CI: https://ci.nodejs.org/job/node-test-pull-request-lite/146/