improve documentation of training_args.py (#8270)

* improve documentation of training_args.py

- do_train
- do_eval
- do_predict

* fix line too long

* fix style with black on training_args.py

* Update src/transformers/training_args.py

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* Update src/transformers/training_args.py

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* Update src/transformers/training_args.py

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* fix line length with utils/style_doc

* black reformatting

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/training_args.py

@@ -48,12 +48,19 @@ class TrainingArguments:
             If :obj:`True`, overwrite the content of the output directory. Use this to continue training if
             :obj:`output_dir` points to a checkpoint directory.
         do_train (:obj:`bool`, `optional`, defaults to :obj:`False`):
-            Whether to run training or not.
+            Whether to run training or not. This argument is not directly used by :class:`~transformers.Trainer`, it's
+            intended to be used by your training/evaluation scripts instead. See the `example scripts
+            <https://github.com/huggingface/transformers/tree/master/examples>`__ for more details.
         do_eval (:obj:`bool`, `optional`):
-            Whether to run evaluation on the dev set or not. Will default to :obj:`evaluation_strategy` different from
-            :obj:`"no"`.
+            Whether to run evaluation on the dev set or not. Will be set to :obj:`True` if :obj:`evaluation_strategy`
+            is different from :obj:`"no"`. This argument is not directly used by :class:`~transformers.Trainer`, it's
+            intended to be used by your training/evaluation scripts instead. See the `example scripts
+            <https://github.com/huggingface/transformers/tree/master/examples>`__ for more details.
         do_predict (:obj:`bool`, `optional`, defaults to :obj:`False`):
-            Whether to run predictions on the test set or not.
+            Whether to run predictions on the test set or not. This argument is not directly used by
+            :class:`~transformers.Trainer`, it's intended to be used by your training/evaluation scripts instead. See
+            the `example scripts <https://github.com/huggingface/transformers/tree/master/examples>`__ for more
+            details.
         evaluation_strategy (:obj:`str` or :class:`~transformers.trainer_utils.EvaluationStrategy`, `optional`, defaults to :obj:`"no"`):
             The evaluation strategy to adopt during training. Possible values are: