DOCS: Clarification on the use of `label_names` as an argument to TrainingArguments (#40353)
* Update trainer.md
* Update trainer.md
Removed the detail about label_names argument usage from the tip/ warning section
* Update training_args.py
Added the label_names usage clarification in the docstring
* Update trainer.md
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Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>