Add Sphinx parameter docs for match and message args to pytest.raises()

[rouge8]

I've been using pytest for years and never noticed either of these until this week!

[nicoddemus]

Thanks a lot @rouge8!

I just renamed the changelog entry to doc and remove the :func: directive because we have some checks which only understand basic RST for now. 👍

[coveralls]

Coverage remained the same at 92.632% when pulling ffee213 on rouge8:pytest-raises-sphinx-parameters into c04e248 on pytest-dev:master.