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?)
@zpao would you like me to add note about `ref` in this PR also? To preserve the ref, should it be copied over explicitly like the key prop?