Clarify modules used for pyproject.toml parsing are private

[abravalheri]

Now that we are stating that configurations passed via pyproject.toml can be considered stable, we should also clarify that the code used to parse/expand/evaluate them are private (see #3232).

In the future we might rename the entire setuptools.config package to setuptools._config.

Summary of changes

For the time being, I am adding an entry to the changelog and a note on the module's docstring.

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)