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
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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/