[ndamania00]

This PR fixes 3 bugs.

1. When parent_link goes from True -> False, it previously would lead to the Update operation setting to None, when it should be False.
2. Liminal type ENTITY_LINK can have parent_link set on it
3. When entity_link goes from something to None, it previously would skip this case. Now, it sets linkDefinition to None and ensures the type remains ANY_ENTITY_LINK_FIELD_DEFINITION if type is unchanged and remains ENTITY_LINK.

Added validation for when bool(parent_link and not entity_link). Tightened validation so it now raises an error if bool(parent_link and type != BenchlingFieldType.ENTITY_LINK)

This PR also moves schema properties docstring to __init__ so that on class hover, it shows up