Update docs to reflect that dispatch() returns a Promise

[rilka]

Was getting a getState() is not a function error when trying to use the async() way of dispatching an async action, and after hunting through the issues I discovered that 02eddbf made all the dispatches return promises (awesome!! ^_^). It was hard to track down; would be awesome if the docs mentioned this somewhere.

Also, I'm a little confused about when one would use async() vs. calling .then() on the result of dispatch. They seem like they're supposed to be the same?

[lexich]

@angelali I add async tools, when redux store.dispatch with can't return promise. And after that, I think that the best way to use .then in most cases. But you are right, that documentation should be updated