Fix doc comment issues reported by pub.dev

[kevmoo]

See

https://pub.dev/packages/async/score/log.txt
https://pub.dev/packages/args/score/log.txt
https://pub.dev/packages/fixnum/score/log.txt

[github-actions]

PR Health

Coverage ✔️

File	Coverage
pkgs/async/lib/src/stream_group.dart	💚 93 %
pkgs/fixnum/lib/src/int32.dart	💚 94 %
pkgs/fixnum/lib/src/int64.dart	💚 98 %

This check for test coverage is informational (issues shown here will not fail the PR).

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.