Update the typing and documentation on the responses parameter of the doc decorator to match the implementation changes introduced in version 2.0.0 via #458.

Fixes #466.

I'm not sure why mypy isn't complaining about the type mismatch, but I think the parameter typing merits expansion after the 2.0.0 changes. I reworded the documentation a bit but I'm open to feedback or adjustments on it.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code docstring.
• Add an entry in CHANGES.md summarizing the change and linking to the issue.
• Add *Version changed* or *Version added* note in any relevant docs and docstring.
• Run pytest and tox, no tests failed.