[async rest] Document how to see skipped tests

Currently, we skip tests for async REST in the following way:

```
if not HAS_ASYNC_REST_EXTRA:
pytest.skip("the library must be installed with the `async_rest` extra to test this feature.")
```

However, the message does not show up when the test is skipped during a nox session.

Investigate and document how to see the skip messages when a test is run in a nox session and skipped using `pytest.skip`. In some cases we should use mark.skipif: https://docs.pytest.org/en/stable/reference/reference.html#pytest-mark-skipif-ref.