ArrowContainer: docs incorrect for targetRect/childRect; arrowSize seems required

[colepeters]

I've found a couple issues using the ArrowContainer on the latest release (6.0.2):

• It looks like the prop that used to be targetRect has been renamed to childRect, but the docs still use targetRect in the API table, and both targetRect and childRect in the examples
• When I don't specify an arrowSize on the ArrowContainer, an arrow is not rendered; it only renders once I provide a size. Was the default perhaps removed?

I would've submitted a PR for the docs change, but since I'm not sure if they also need to change based on the arrowSize, I'll leave it for now!