[DOC] Clarify relationshi load_best_model_at_end and save_total_limit
Clarify the relationship between `load_best_model_at_end` and `save_total_limit`. Hope this is clear.

As discussed on Slack @sgugger 

_The documentation is not available anymore as the PR was closed or merged._<|||||>Can you just do a quick rebase/merge on the main branch? I think the issue in the tests (due to a release of Pillow apparently) is fixed on main.