add evaluate doc - trainer.evaluate returns 'epoch' from training (#8273)

* add evaluate doc

* fix style with utils/style.doc

* Update src/transformers/trainer.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/trainer.py

@@ -1297,7 +1297,8 @@ class Trainer:
                 :obj:`__len__` method.
 
         Returns:
-            A dictionary containing the evaluation loss and the potential metrics computed from the predictions.
+            A dictionary containing the evaluation loss and the potential metrics computed from the predictions. The
+            dictionary also contains the epoch number which comes from the training state.
         """
         if eval_dataset is not None and not isinstance(eval_dataset, collections.abc.Sized):
             raise ValueError("eval_dataset must implement __len__")