text: Title: Document which environment variables are passed through by default
  Body: Documentation for tox3 had https://tox.wiki/en/3.27.0/config.html#conf-passenv
  
  Documentation for tox4 does not show the list any more
  
  Also see https://github.com/tox-dev/tox/blob/6b1cc141aeb9501aa23774056fbc7179b719e200/src/tox/tox_env/api.py#L179-L204