[trainer] document resume randomness (#11588)

* document resume randomness

* fix link

* reword

* fix

* reword

* style

docs/source/main_classes/trainer.rst

@@ -119,6 +119,20 @@ TFTrainingArguments
     :members:
 
 
+Randomness
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When resuming from a checkpoint generated by :class:`~transformers.Trainer` all efforts are made to restore the
+`python`, `numpy` and `pytorch` RNG states to the same states as they were at the moment of saving that checkpoint,
+which should make the "stop and resume" style of training as close as possible to non-stop training.
+
+However, due to various default non-deterministic pytorch settings this might not fully work. If you want full
+determinism please refer to `Controlling sources of randomness
+<https://pytorch.org/docs/stable/notes/randomness.html>`__. As explained in the document, that some of those settings
+that make things determinstic (.e.g., ``torch.backends.cudnn.deterministic``) may slow things down, therefore this
+can't be done by default, but you can enable those yourself if needed.
+
+
 Trainer Integrations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~