Document version 4 different behaviours

[jugmac00]

tox 3 behavior

❯ tox -e py3 tests/unittests/net/test_networkd.py

... works

tox 4 behavior

❯ tox4 -e py3 tests/unittests/net/test_networkd.py
usage: tox [-h] [--colored {yes,no}] [-v | -q] [-c file] [--workdir dir] [--root dir] [--runner {virtualenv}] [--version] [--no-provision [REQ_JSON]] [--no-recreate-provision] [-r] [-x OVERRIDE]
           {run,r,run-parallel,p,depends,de,list,l,devenv,d,config,c,quickstart,q,exec,e,legacy,le} ...
tox: error: unrecognized arguments: tests/unittests/net/test_networkd.py
hint: if you tried to pass arguments to a command use -- to separate them from tox ones

While I think the behavior for tox 4 is more correct, at least we need to document this incompatibility.

Repository for above snippets:
https://github.com/canonical/cloud-init

tox configuration
https://github.com/canonical/cloud-init/blob/7a93f1cad75715696227aa0893ff15156eaa7943/tox.ini#L140-L147

[gaborbernat]

The hint after the error is the documentation itself, what more you want?

[jugmac00]

I want a dedicated section in our documentation about expected incompatibilities, which a user can read in advance, so no trial and error is necessary. This might be a standalone section or part of the release notes for tox 4 - though I think a standalone section would make more sense as I expect this list will grow bigger once tox 4 gets more widely used.

[gaborbernat]

You're free to open a PR for this 👌 I'll not make it a 4.0 release blocker though.