Fix docs for React.Children.map, .forEach, .only.

- `React.Children.map` returns an object, not an array (!)
- `React.Children.only` throws if not a single valid component

I assume returning null or undefined from [`mapChildren`](https://github.com/facebook/react/blob/141f3a8/src/utils/ReactChildren.js#L116-L118) is intentional, but there's also a bit strange null check and return in [`forEachChildren`](https://github.com/facebook/react/blob/141f3a8/src/utils/ReactChildren.js#L60-L62), even though it doesn't ordinarily return anything and `traverseAllChildren` would handle it in any case?