How to make an input parameter/flag default to a named Environment Variable?

As the title says;

- I want a flag/input parameter named `--foo-bar`. (No probs there).
- I want `--help` to show that `--foo-bar` gets its default value from environment variable `FOO_BAR`.
(Maybe also show the value of the current envvar `FOO_BAR` if it exists in current shell/machine)?

issue 中没有列出文件、测试或入口点。先定位选项默认值和帮助生成的处理路径，然后明确环境变量是否应提供默认值，以及它们的当前值是否应显示在帮助中。完成的标准是行为和显示格式已明确规定，并且有测试覆盖。

由索引模型根据 Issue 内容生成。