doc: document --trace-promises

[tomass22]

--trace-promises has been available since v21.5.0 (backported to v20.12.0 in
#50899) but has never been documented.

• Registered in src/node_options.cc with kAllowedInEnvvar, so it is also
  usable through NODE_OPTIONS
• Implemented in src/env.cc as Environment::TracePromises, installed as an
  isolate-wide promise hook in src/node.cc
• Absent from both doc/api/cli.md and doc/node.1

When the flag landed, it was added to the "intentionally undocumented options"
list in test/parallel/test-process-env-allowed-flags-are-documented.js instead
of being documented:

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

I believe that was an oversight rather than a deliberate decision. The other
entries in that list fall into two groups:

• legacy no-ops kept for compatibility — --experimental-fetch,
  --experimental-global-webcrypto, --experimental-report,
  --experimental-worker, --napi-modules, --es-module-specifier-resolution
• internal-only debugging switches — --debug-arraybuffer-allocations,
  --verify-base-objects, --node-snapshot

--trace-promises is neither. It is a user-facing diagnostic flag in the same
family as --trace-env, --trace-uncaught and --trace-warnings, all of which
are documented.

This PR:

• documents the flag in doc/api/cli.md, both as a ### section and in the
  NODE_OPTIONS allow-list
• regenerates doc/node.1 with make node.1
• removes the two exemptions from the test

No new test is needed. Removing the exemptions makes the existing
test-process-env-allowed-flags-are-documented test cover this flag — it fails
without the documentation change.

Refs: #50899