Improve documentation around generated Haskell code

Document the mapping between C-side and Haskell-side types.
Document the relationship between MyClass/Ptr MyClass/Ptr (Ptr MyClass), when each is used, and how to convert. Consider referring to MyClass as a handle to differentiate it.
Document the specific meanings of encode, decode, and assign (since the latter two are similar but not fully equivalent to peek and poke).