refactor: update sync model helper function docs and minor cleanup (openedx#36599) (openedx#36645)

* refactor: update sync model helper function docs

Adds some comments to explain certain confusing sections

* refactor: sync library content method

* feat: use edited_on block field

* test: modified field in course block index

* fix: extract uncommon methods to child class from base

(cherry picked from commit c2d9e40)

Author: navinkarkera