doc: `--trace-promises` is not documented in the CLI reference

[tomass22]

Affected URL(s)

https://nodejs.org/api/cli.html

Description of the problem

--trace-promises is implemented and shipped, but it is not documented anywhere
in the CLI reference.

• Registered in src/node_options.cc with kAllowedInEnvvar, so it can also be
  set through NODE_OPTIONS.
• Implemented in src/env.cc as Environment::TracePromises, installed as an
  isolate-wide promise hook in src/node.cc.
• Shipped in v21.5.0 and backported to v20.12.0 (src: implement --trace-promises nodejs/node#50899).

It appears in neither doc/api/cli.md nor doc/node.1. When the flag was added,
it was instead placed in the "intentionally undocumented options" list in
test/parallel/test-process-env-allowed-flags-are-documented.js, which is what
keeps that test passing today:

assert(undocumented.delete('--trace-promises'));
assert(undocumented.delete('--no-trace-promises'));

Every sibling tracing flag is documented: --trace-env, --trace-uncaught,
--trace-warnings, --trace-exit, --trace-sync-io. The other entries in that
exemption list are either legacy no-ops (--experimental-fetch,
--napi-modules) or internal-only debugging switches
(--verify-base-objects, --debug-arraybuffer-allocations).
--trace-promises is neither, so this looks like an oversight rather than a
deliberate omission.

I would like to send a PR documenting it and removing the exemption.