#1088 added text for caching frame and viewport objects.

It's not ideal to talk about "the previous object returned by the call", as mentioned by @cabanier . However, since both of these are caching a limited number of objects, we can explicitly cache these objects somewhere, e.g. we can have an "animation frame" stored on the XRSession and a "viewport object" stored on the XRView.

This would not be a behavior change, this would just make existing behavior more explicit.