lib: comment explaining special-case handling of promises

Promise handling is special-cased for domains and trace_events.
Domains must not add the `domain` property to promises in other
contexts, and trace_events must be emitted from JavaScript as
promises no longer have an AsyncWrap type attached to them.

PR-URL: https://github.com/nodejs/node/pull/39135


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>