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

Please review the following changes:
  * 43e80bc Data model docs: Update the "Deserialization from JSON" section
  * ee77839 Data model docs: Describe advantages of OO-based model better
  * 91c8481 Data model docs: Be more precise about classes of non-leaf nodes