dillon1000/react

docs better indicate that state updaters shallowly merge with state (#9554)

this was a surprise to me because the docs seemed to indicate that when
using an updater, the result _needed_ to be a new state object. I was
[not alone](https://twitter.com/ken_wheeler/status/857939690191806464)
i think in discovering this as a result of the previous tweet in the
thread.