"Conditions as strings instead of booleans" docs act like `pytest.config` is still a thing

[jwodder]

The documentation on "Conditions as strings instead of booleans" ends by showing how to rewrite a condition string that uses config as a boolean expression that uses pytest.config. However, as far as I can tell, pytest.config was removed in Pytest 5.1, yet the documentation makes no mention of this, and thus readers may be mislead into thinking that the "modern equivalent" of condition strings can be achieved by using pytest.config.