Add a comment with known differences from the spec to the docstring

Author: asmeurer

numpy_array_api_compat/__init__.py

@@ -16,6 +16,16 @@
 to ensure they are not using functionality outside of the standard, but prefer
 this implementation for the default when working with NumPy arrays.
 
+Known differences from the Array API spec:
+
+- The array methods __array_namespace__, device, to_device, and mT are not
+  defined. This reuses np.ndarray and we don't want to monkeypatch or wrap it.
+
+- NumPy value-based casting for scalars will be in effect unless explicitly
+  disabled with the environment variable NPY_PROMOTION_STATE=weak or
+  np._set_promotion_state('weak') (requires NumPy 1.24 or newer, see NEP 50
+  and https://github.com/numpy/numpy/issues/22341)
+
 """
 
 from numpy import *