Update doc for `metric_for_best_model` when `save_strategy="best"`.
Author: seanswyi
Pull request: yes

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Updates the docstring for `TrainingArguments.metric_for_best_model`, `Trainer._determine_best_metric`, and adds a new test.

Specifically, when `save_strategy="best"` we need to specify a value for `metric_for_best_model`. This clashes with the previous logic that `metric_for_best_model` would default to loss.

Brought up in this comment: https://github.com/huggingface/transformers/pull/31817#discussion_r1891105193


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@muellerzr @SunMarc (cc. @shcheklein - Author of comment)

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Comments:
@SunMarc Made some updates to the docstring for `metric_for_best_model` as per your suggestion. I changed it up a bit so that it's more descriptive than the original message. Please let me know if there's anything more to be done.