`_secret_questions` setting is undocumented

[sisp]

The _secret_questions setting

https://github.com/copier-org/copier/blob/608654a03e748a3ed1b032c089e20b2811a0cfd3/tests/test_answersfile.py#L34-L35

is not documented. I think it's a redundant setting since a question supports the secret flag as well. In any case, the docs and capabilities should be in sync.

Which solution do you prefer?

1. Document this setting.
2. Remove support for this setting in the code.