Cleanup documentation for BART, Marian, MBART and Pegasus
# What does this PR do?

This is a follow-up from #7345 to finishing cleaning up the documentation for all models. Nothing of importance apart from the configurations of the 4 classes (BART, Marian, MBART and Pegasus) that can't have the same docstrings with a choose your own adventure default. They all need their own docstrings, since the defaults are different for a lot of values (which also means they all need their own implementation and not subclass the same config or at least pass the arguments with the proper defaults to the superclass).

I tried my best to document the actual default for each, but may missed a few fields.