python docs: improve override example

1. Use the same approach like in the overlay example:
   Override `python` instead of `pythonPackages` so that
   `python.pkgs` refers to the new package set like `pythonPackages`.

   This also fixes a bug in the original example where
   `pkgs.fetchgit` was not in scope.

   Add an extra example to illustrate how to override just a
   package set.

2. Fix mix-up between `super` and `self` in the explanation text.
   Also, simplify the explanation.

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