Document which environment variables are passed through by default

[seyidaniels]

Fixes https://github.com/tox-dev/tox/issues/2753

• [ ] 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