docs: update note

Author: kgryte

src/array_api_stubs/_draft/manipulation_functions.py

@@ -59,7 +59,7 @@ def broadcast_shapes(*shapes: Tuple[int, ...]) -> Tuple[int, ...]:
     -----
 
     -   If not provided one or more arguments, the function **must** return an empty tuple.
-    -   Array libraries which build computation graphs (e.g., ndonnx and Dask) commonly support shapes having dimensions of unknown size. If a shape contains a value other than an integer (e.g., ``None`` for a dimension of unknown size), array-conforming libraries **must** propagate such values (e.g., if a shape contains a dimension size of ``None``, the returned broadcasted shape **must** also have a corresponding dimension having a size equal to ``None``).
+    -   An array-conforming library **may** accept a non-integer value (e.g., ``None``) for one or more dimensions. This is common for array libraries which build computation graphs (e.g., ndonnx and Dask) and which need to provide a mechanism for indicating dimensions of unknown size. Array-conforming libraries which accept non-integer values for dimensions of unknown size **must** propagate such values (e.g., if a shape contains a dimension size of ``None``, the returned broadcasted shape **must** also have a corresponding dimension having a size equal to ``None``); otherwise, an array-conforming library **must** raise a ``ValueError``.
     """