Reconciliation docs need amendment

[rscharfer]

I think it can be clarified that the component that's rendering a React element of a new type will itself not be remounted; however, its child elements will be.

In other words if a component's render method returns

<div>
  <Counter />
</div>

when it is mounted and later (when it is updated)

<span>
  <Counter />
</span>

that component itself will not be remounted, but the <Counter/> element will be.

[rscharfer]

https://codesandbox.io/s/v0xxp37pql?fontsize=14