Fix `TrainingArguments.label_names` docs to reflect the correct default value behaviour (#21288)

* Update TrainingArguments.label_names docs

* Change wording

* Change wording

src/transformers/training_args.py

@@ -352,8 +352,9 @@ class TrainingArguments:
         label_names (`List[str]`, *optional*):
             The list of keys in your dictionary of inputs that correspond to the labels.
 
-            Will eventually default to `["labels"]` except if the model used is one of the `XxxForQuestionAnswering` in
-            which case it will default to `["start_positions", "end_positions"]`.
+            Will eventually default to the list of argument names accepted by the model that contain the word "label",
+            except if the model used is one of the `XxxForQuestionAnswering` in which case it will also include the
+            `["start_positions", "end_positions"]` keys.
         load_best_model_at_end (`bool`, *optional*, defaults to `False`):
             Whether or not to load the best model found during training at the end of training.