Wrong return type in docs for useMutation

[nicoqh]

According to the docs, the error returned by useMutation can either be null or Error.

However, it can also be undefined when mutationReducer returns a new state object without the error property. types/test.ts suggests that this is intended.

If so, the docs should say null | undefined | Error.

I'm opening an issue instead of a PR for the docs, in case you want to change the reducer instead.