MBart: Fix docs and doctests
# What does this PR do?

Related issue: #22397 

Most fixes are on the TF side. Elaborated a sentence in the main docstring -- MBart can be used for summarization, but none of the pre-trained models were trained for summarization.

Alongside this PR, TF checkpoints for MBart are being pushed to the Hub 🤗 
_The documentation is not available anymore as the PR was closed or merged._