Update docs about CUDA stream priority (pytorch#41364)

Summary: Pull Request resolved: pytorch#41364

Reviewed By: malfet

Differential Revision: D22962856

Pulled By: ngimel

fbshipit-source-id: 47f65069516cb555579455e8680deb937fc1f544

torch/cuda/streams.py

@@ -20,8 +20,12 @@ class Stream(torch._C._CudaStreamBase):
         device(torch.device or int, optional): a device on which to allocate
             the stream. If :attr:`device` is ``None`` (default) or a negative
             integer, this will use the current device.
-        priority(int, optional): priority of the stream. Lower numbers
-                                 represent higher priorities.
+        priority(int, optional): priority of the stream. Can be either 
+            -1 (high priority) or 0 (low priority). By default, streams have
+            priority 0.
+
+    .. note:: Although CUDA versions >= 11 support more than two levels of
+        priorities, in PyTorch, we only support two levels of priorities.
     """
 
     def __new__(cls, device=None, priority=0, **kwargs):