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

Please review the following changes:
  * ae4e415 Data model docs: Update the "Deserialization from JSON" section
  * 63ee17a Data model docs: Describe advantages of OO-based model better
  * 3d2c680 Data model docs: Be more precise about classes of non-leaf nodes