Improve documentation for Stack.Runners

[sjakobi]

For #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.

[mgsloan]

Agreed, this is something that sometimes puzzles me, so would be a great thing to document.

One way to make things clearer might be to use a concrete monad in more places: #1297