update docs (#4739)

docs/source/trainer.rst

@@ -1475,6 +1475,11 @@ with the hidden
             # hiddens are the hiddens from the previous truncated backprop step
             out, hiddens = self.lstm(data, hiddens)
 
+            # remember to detach() hiddens.
+            # If you don't, you will get a RuntimeError: Trying to backward through
+            # the graph a second time...
+            # Using hiddens.detach() allows each split to be disconnected.
+
             return {
                 "loss": ...,
                 "hiddens": hiddens  # remember to detach() this
@@ -1702,4 +1707,3 @@ The metrics sent to the progress bar.
 
     progress_bar_metrics = trainer.progress_bar_metrics
     assert progress_bar_metrics['a_val'] == 2
-