[Contributing] Clarify the rebase operation

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | N/A

I just mentioned the fact that `push -f` is usually required when rebasing and explicit that the "2.2 when bugfixing" is a follow up on the previous example (Not necessarily explicit at first).