This rule should prevent usage of this.state inside setState calls.
Such usage of this.state might result in errors when two state calls is
called in batch and thus referencing old state and not the current
state. An example can be an increment function:

function increment() {
  this.setState({value: this.state.value + 1});
}

If these two setState operations is grouped together in a batch it will
look be something like the following, given that value is 1.

setState({value: 1 + 1})
setState({value: 1 + 1})

This can be avoided with using callbacks which takes the previous state
as first argument. Then react will call the argument with the correct
and updated state, even when things happen in batches. And the example
above will be something like.

setState({value: 1 + 1})
setState({value: 2 + 1})