Improve documentation for Stack.Runners

For https://github.com/commercialhaskell/stack/issues/2692 I had to navigate the various runners and configuration types again.

This would have been easier if the module explained e.g. that the `withXConfig` functions are ordered by "power" and what information they carry in addition to the previous type. `MiniConfig` is the "largest" type that doesn't require an existing GHC installation to construct etc.