The document fails to explain what is meant by asset ID, how different images are returned based on the theme and modifier, or a list of possible assets, if that’s how it works. Furthermore, the “MoreButton” example returns an empty string.
The entire document is generally vague, and seems to assume that the reader has knowledge of some other feature that it fails to link to, possibly because such information does not exist. It is also possible that this API is not meant to be used at all.