Add StateT and trampolining to State documentation

[ceedubs]

Documentation for State was added in #384. However, it still has a TODO at the end for documenting how StateT relates to State and how/why trampolining is used for State. This TODO should be replaced with documentation.

[LukaJCB]

Maybe we should explain the concept of Monad Transformers at some sort of central location? Some of the motivation and explanation already exists in the OptionT documentation. We could take some of that and make some sort of Monad Transformer section, or we duplicate some of the documentation, which I'd personally also be okay with.

Another thought I have, is that cats-mtl is going to be a different package soon AFAIK, so there might also be some duplication there.

[edmundnoble]

cats-mtl will provide documentation about monad transformers in general, yes.

Edit: Because it will not provide the transformers themselves, and not the type classes, it may be a bit unusual to document them in cats-mtl. What do others think?

[kailuowang]

related #1714
TLDR, we may need a reformulation of StateT.