print-deps-only: Document we don't handle recursive deps (from files, e.g. -rrequirements.txt)

Fedora report: https://bugzilla.redhat.com/show_bug.cgi?id=1808601

tox docs: https://tox.readthedocs.io/en/latest/example/basic.html#depending-on-requirements-txt-or-defining-constraints

When the dependencies are defined as:

```
[testenv]
deps = -rrequirements.txt
```

We need to read the files (recursively) and output their content.

The code that handles this now is https://github.com/fedora-python/tox-current-env/blob/dc47ce3ddf53c2b950f0a776ea6ebcc7355e3522/src/tox_current_env/hooks.py#L161-L172

The `*venv.get_resolved_dependencies()` call needs to be replaced with a custom function call. That function needs to call `venv.get_resolved_dependencies()` and handle "packages" that start with `-r`/`-c` specially.

- https://pip.pypa.io/en/stable/user_guide/#requirements-files
- https://pip.pypa.io/en/stable/user_guide/#constraints-files

Read src/tox_current_env/hooks.py around lines 161-172 and inspect the current venv.get_resolved_dependencies() call. Check the tox and pip requirements-file documentation, then trace how -r and -c dependency entries are represented. Done means print-deps-only recursively reads those files and outputs their contents, including nested references.

索引モデルが issue の本文から書いたものです。