Document check copies
# What does this PR do?

This PR document a little bit better how or `Copied from` framework works, adds comments in the actual scripts and rework a bit the test to be better.

In passing I added a feature requested which was to make sure `make fix-copies` took the function definition or the superclass into account: currently it ignore the whole first line, but if we change the signature of a function / the superclass of a class which is copied from, that modification is not propagated (cc @Rocketknight1 who last requested it)

As you can see from the diff, that feature was direly needed... I had to add `BartPreTrainedModel` (right spelling to be consistent with other models) or break multiple copies, and you can see a lot of signatures or copied from statement being fixed.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25291). All of your documentation changes will be reflected on that endpoint.