[CDEC-385] deriveSimpleBi: update error message/docs
[CDEC-385] deriveSimpleBi: update error message/docs

- Spruce up example derivations for `deriveSimpleBi` and
  `deriveIndexedBi`.
- Document `deriveIndexedBi` & the motivation behind it.

input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl