Standardize "store" vs "cache"

[lorensr]

If they mean different things, it would be helpful to introduce that somewhere in the docs. If they mean the same thing, let's standardize on one term? Store is more familiar than cache to Reduxers. But outside of Redux context, I think store has more of a persistent connotation than cache (it's where you store data—like, permanently), and Apollo's store is usually not persistent.