[RAC] DatePicker `onChange` prop is not documented

[agos]

🙋 Documentation Request

I noticed this because the type of the onChange prop changed to include null but on the documentation page it's not mentioned when null will be passed to the callback, or anything else about it really

🧢 Your Company/Team

No response

[devongovett]

It's null when the date is cleared (e.g. backspace). This has always been the case but the types were wrong before.