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)?

No files, tests, or entry points are named in the issue. Start by locating the option-default and help-generation paths, then clarify whether environment variables should supply defaults and whether their current values should appear in help. Done means the behavior and display format are specified and covered by tests.

Written by the indexing model from the issue text.