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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