Breaking model

Apply text provided by @mtbc regarding breaking changes to the 5.0 model: namely `OriginalFile.sha1` --> `OriginalFile.hash`. Since there was no clear landing page for putting the breaking info, I put some under `whatsnew` and the details with the matching documentation.
I think the `experimental` marker takes care of that.  If it's non-functional, I'd argue it shouldn't yet be in the docs unless it's marked "for future use" or equivalent.