Add more explicit language on how clients should process items re: order

[kirschbombe]

The spec states that “Clients should process the Annotation Pages and their items in the order given in the Canvas.” (https://iiif.io/api/presentation/3.0/#55-annotation-page). In the case of two or more images layered onto a single anno page (i.e choice or composition from multiple images), this language might be too vague if the expectation is that the images are assembled upwards on the canvas in the order they appear in the manifest. Do we want to make this explicit or are there reasons to leave it more open?