gh-62480: De-personalize "Partial mocking" section in `unittest.mock` examples (#141321)

* Refine some wording in unittest partial mock doc

Some of the descriptions were addressed in first person,
but have now been changed to address the user reading the documentation instead.

Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>