Clarify that actx.array_types allows ABCs

Author: inducer

arraycontext/context.py

@@ -75,8 +75,8 @@
 
 .. currentmodule:: arraycontext
 
-The interface of an array context
----------------------------------
+The :class:`ArrayContext` Interface
+-----------------------------------
 
 .. autoclass:: ArrayContext
 
@@ -278,8 +278,10 @@ class ArrayContext(ABC):
 
         A :class:`tuple` of types that are the valid array classes the
         context can operate on. However, it is not necessary that *all* the
-        :class:`ArrayContext`\ 's operations would be legal for the types in
-        *array_types*.
+        :class:`ArrayContext`\ 's operations are legal for the types in
+        *array_types*. Note that this tuple is *only* intended for use
+        with :func:`isinstance`. Other uses are not allowed. This allows
+        for 'types' with overridden :meth:`class.__instancecheck__`.
 
     .. automethod:: freeze
     .. automethod:: thaw