Clarify rules for `children` in component invocations

children are now treated like all other attributes during component invocation:

1. If there's an explicit children parameter, we pass in a tuple. If there are no children in the underlying HTML, the tuple will be empty.
2. If there's a **kwargs parameter, we always include children; the component is free to ignore this key.
3. If there's no children parameter and no **kwargs, we ignore children in the underlying HTML if there are any.