Data model docs: Be more precise about classes of non-leaf nodes
Data model docs: Describe advantages of OO-based model better

Previous description came from a time when everything was arrays &
OpenStructs and was no longer accurate.
Data model docs: Update the "Deserialization from JSON" section

Remove the JSON Schema stuff (not implemented yet), update the rest of
the text to describe the current state.
Merge pull request #75 from SUSE/data-model-docs-more-improvements-2

[Review] Request from 'dmajda' @ 'SUSE/machinery/data-model-docs-more-improvements-2'