Review and tweak Image API change-log. Minor changes only

Document looks very good. I've made a number of typo fixes, a few rewordings, no substantive changes unless I misunderstood intention of original:

* The formats in which the image is available are explicitly enumerated by the info.json document for an image and/or the server compliance level document. A client should never need to guess or let the server decide as `jpg` is always required.

which I split into:

* The formats in which the image is available are explicitly enumerated by the `info.json` document for an image and/or the server compliance level document. A client should never need to guess or let the server decide.
* The `jpg` format must be supported at all compliance levels and thus applications requiring it can rely upon its availability.