Adds a note mentioning that DatasetIterator is not thread-safe.

PiperOrigin-RevId: 698752774

grain/_src/python/dataset/dataset.py

@@ -1037,7 +1037,11 @@ def __iter__(self) -> DatasetIterator[T]:
 
 
 class DatasetIterator(Iterator[T], abc.ABC):
-  """`IterDataset` iterator."""
+  """`IterDataset` iterator.
+
+  NOTE: The methods are assumed to be thread-unsafe. Please ensure only a single
+  thread can access a `DatasetIterator` instance.
+  """
 
   # Whether this transformation mutates parent elements. This does not affect
   # the transformation itself, only used for information purposes in statistics.