docs: add note regarding unspecified behavior in cumulative_sum

Fixes: data-apis#797

src/array_api_stubs/_draft/statistical_functions.py

@@ -18,7 +18,7 @@ def cumulative_sum(
     Parameters
     ----------
     x: array
-        input array. Should have a numeric data type.
+        input array. Should have one or more dimensions (axes). Should have a numeric data type.
     axis: Optional[int]
         axis along which a cumulative sum must be computed. If ``axis`` is negative, the function must determine the axis along which to compute a cumulative sum by counting from the last dimension.
 
@@ -48,6 +48,8 @@ def cumulative_sum(
     Notes
     -----
 
+    -   When ``x`` is a zero-dimensional array, behavior is unspecified and thus implementation-defined.
+
     **Special Cases**
 
     For both real-valued and complex floating-point operands, special cases must be handled as if the operation is implemented by successive application of :func:`~array_api.add`.