document strict-config and other command line flags

[graingert]

I was recently asked to explain why I was adding --strict-config to a pytest ini, "as it appears to be undocumented"

currently command line flags are only documented in the changelog and https://docs.pytest.org/en/stable/_modules/_pytest/main.html

I think we should automatically add the output of pytest --help to the docs, eg here:
https://docs.pytest.org/en/stable/usage.html#getting-help-on-version-option-names-environment-variables

[Zac-HD]

Sounds good to me. I've used sphinxcontrib-programoutput for this before, if nobody has a better idea?

[The-Compiler]

If we just want the raw output: We already do use regendoc all over the docs, so without a good reason I don't think we should add yet another way to generate command output in the documentation.