[DOCS] state - virtualenv.managed - onedir documentation needed + possibly block this state

[ITJamie]

Description
I just noticed another place where we should add warnings about the python onedir change.
when using the virtualenv.managed state with salt onedir. if you dont supply the python variable it will use onedir’s python to create the virtualenv https://docs.saltproject.io/en/latest/ref/states/all/salt.states.virtualenv_mod.html#salt.states.virtualenv_mod.managed
we called needing to update pip.installed uses for the onedir changeover, but we should also add virtualenv.managed to those warnings
https://docs.saltproject.io/salt/install-guide/en/latest/topics/upgrade-to-onedir.html#how-to-upgrade-to-onedir (step 4)

Suggested Fix
add docs on upgrade guide to supply python path to a system python package

Type of documentation
Salt modules + salt repo / salt install guide

Additional context
It might be a good idea to block use of virtualenv.managed on onedir builds if python var isnt supplied. aka making it a required field

[OrangeDog]

Supplying venv_bin should also be sufficient.

[barbaricyawps]

Thanks again for opening this issue, @ITJamie . I'll assign myself to this issue and try to get some doc updates for you.