Improve `ParentSync` docs

[Shatur]

• Hide the implementation details about None because the field does not appear in docs and confuses users.
• Add a usage example.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.01%. Comparing base (5325a47) to head (2321702).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #274 +/- ## ======================================= Coverage 91.01% 91.01% ======================================= Files 38 38 Lines 2327 2327 ======================================= Hits 2118 2118 Misses 209 209 ```

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