Add note about key prop being excluded by cloneWithProps

Whilst this is intentional behaviour (see #1274), it is not documented anywhere, so it is worth mentioning.

It would also be nice if React issued a warning to console if a cloned component loses its key (as this will silently break reconciliation?)
Good point. Can you use the blockquote formatting we have for notes

```md
> Note:
>
> blah blah blah
```