docs: default_device() can be None

PR-URL: #961
Co-authored-by: Athan Reines <kgryte@gmail.com>
Reviewed-by: Athan Reines <kgryte@gmail.com> 
Reviewed-by: Jake Vanderplas
Reviewed-by: Tim Head

Author: ev-br

src/array_api_stubs/_draft/info.py

@@ -80,11 +80,13 @@ def default_device() -> device:
     Returns
     -------
     out: device
-        an object corresponding to the default device.
+        an object (including ``None``) corresponding to the default device.
 
     Notes
     -----
 
+    -   A conforming array library may return ``None`` if the default device is not predictable due to library specific device placement rules.
+
     .. versionadded: 2023.12
     """