Missing injected environment variables in tox 4 documentation

[xymy]

The documentation for tox 3.x introduced injected environment variables.

However, there is almost no information about them except release history in tox 4.x documentation. Any chance to supplement their documentation?

[jugmac00]

I think that is a great idea.

[ssbarnea]

@xymy Nice catch, I found myself using google a lot to find them as sometimes I need them but I always forget their names. Make PR to document them, that would a nice first contribution.