There are a few notes that could be included here, although they don't fit the style of introductory documentation so well. Nevertheless they may still be worth including.

• file(s) on disk part is not always true in the case of packed and generated images.
• In what situations one images references multiple ImBuf's (UDIM, multi-view/stereo, animation... etc).

There is room for some more detailed examples too, something that involved loading, modifying and saving an image would be good. Although that can be separate from this patch (another example file).