Add documentation for array interface protocol deprecation

CHANGES

@@ -4,6 +4,11 @@ Pint Changelog
 0.10 (unreleased)
 -----------------
 
+- Deprecate array protocol fallback except where explicitly defined (`__array__`,
+  `__array_priority__`, `__array_function__`, `__array_ufunc__`). The fallback will
+  remain until the next minor version, or if the environment variable
+  `PINT_ARRAY_PROTOCOL_FALLBACK` is set to 0.
+  (Issue #953, Thanks Jon Thielen)
 - Removed eval usage when creating UnitDefinition and PrefixDefinition from string.
   (Issue #942)
 - Added `fmt_locale` argument to registry.

docs/numpy.rst

@@ -159,6 +159,15 @@ memory and CPU cycles. Therefore, for numerically intensive code, you
 might want to convert the objects first and then use directly the magnitude,
 such as by using Pint's `wraps` utility (see :ref:`wrapping`).
 
+Array interface protocol attributes (such as `__array_struct__` and
+`__array_interface__`) are available on Pint Quantities by deferring to the
+corresponding `__array_*` attribute on the magnitude as casted to an ndarray. This
+has been found to be potentially incorrect and to cause unexpected behavior, and has
+therefore been deprecated. As of the next minor version of Pint (or when the
+`PINT_ARRAY_PROTOCOL_FALLBACK` environment variable is set to 0 prior to importing
+Pint), attempting to access these attributes will instead raise an AttributeError.
+
+