📝 docs(config): fix five wrong defaults in the reference - #4114

The configuration reference gave the wrong default for five settings, so readers expected behavior tox doesn't have. 📝 tox config prints the real values for a project that sets none of them:

$ tox config -e py,.pkg -k args_are_paths pkg_dir
[testenv:py]
args_are_paths = True

[testenv:.pkg]
pkg_dir = /path/to/project/.tox/.pkg/dist

$ tox config --core -e py -k skip_missing_interpreters min_version requires
[testenv:py]
skip_missing_interpreters = False

[tox]
skip_missing_interpreters = False
min_version = None
requires = tox

The reference now lists True for args_are_paths (it said False), {env_dir}/dist for pkg_dir (it said {env_dir}/.dist), False for skip_missing_interpreters (it said config), no minimum for min_version (it said the running tox version), and tox for requires (it said an empty list). The requires entry now says tox always adds itself to the list. The skip_missing_interpreters entry no longer suggests writing config in the file, which fails with an error; config is a value for the --skip-missing-interpreters flag.

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation