State None as a possible value of stream argument in docstrings

Author: antonwolfy

dpnp/dpnp_array.py

@@ -1698,7 +1698,7 @@ def to_device(self, device, /, *, stream=None):
             corresponding to a non-partitioned SYCL device, an instance of
             :class:`dpctl.SyclQueue`, or a :class:`dpctl.tensor.Device` object
             returned by :obj:`dpnp.dpnp_array.dpnp_array.device` property.
-        stream : SyclQueue, optional
+        stream : {SyclQueue, None}, optional
             Execution queue to synchronize with. If ``None``, synchronization
             is not performed.
             Default: ``None``.