text: Title: [docs] Add a verbose explanation on how `tox exec` works
  Body: This docs section https://tox.wiki/en/latest/cli_interface.html#tox-exec-(e) does not actually show how to pass a command in (it's not in the signature). And it doesn't explain if the default commands are executed. It documents `--notest` in the signature, which makes it confusing — does the passed command replace the defined ones or does it augment them? What about `commands_pre`?
  
  From what I saw in the wild (https://github.com/jamescooke/flake8-aaa/blob/master/Makefile#L42C2-L42C10), the command to execute should be passed after `--`. But that's a typical syntax for `{posargs}`. So how do these parts play together?