problem

Some environment variables to configure tox are undocumented as far as I searched on readthedocs. Some examples are:

• TOX_DISCOVER, which is in changelog 3.14.5 but not elsewhere. I searched in this repository for an environment-variable alternative to --discover command line option to find this environment variable.
• TOX_PROVISION

and others can be found by searching.

possible improvement

1. There is an Injected environment variables section in tox configuration specification. I found it is most straightforward to put them into this section. Since they don't seem to be environment variables "that you can use to test that your command is running within tox", the paragraph can also be rewritten.
2. Since TOX_DISCOVER corresponds to --discover, it is also possible to add it to help argument of parser.add_argument calls`. I haven't check if this applies to all the undocumented environment variables.