Update parameters.yml documentation to talk about .env

[weaverryan]

Changes to be made for 4.0.

[javiereguiluz]

I created a meta-issue for the changes needed for 4.0 (#7809) but maybe it's better to create smaller issues like this one? My fear with meta-issues is that they can end up being never-ending and too big.

[weaverryan]

Ok, that’s great! I don’t mind having the meta issue - it’s nice. So let’s not automatically create issues for everything. But if some items do have their own issue, we can just link. Until we have done more work and are closer to ready, I don’t think we should worry too much about perfectly organizing things.

[weaverryan]

After #8779 and once #8754 is done (specifically, the quick tour), this will be done. So, I'm going to close it.