docs: Document __objclass__. Closes #19281.

Initial patch by Nick Coghlan

Doc/reference/datamodel.rst

@@ -1467,6 +1467,14 @@ class' :attr:`__dict__`.
    Called to delete the attribute on an instance *instance* of the owner class.
 
 
+The :attr:`__objclass__` is interpreted by the :mod:`inspect` module as
+specifying the class where this object was defined (setting this appropriately
+can assist in runtime introspection of dynamic class attributes). For callables,
+it may indicate that an instance of the given type (or a subclass) is expected
+or required as the first positional argument (for example, CPython sets this
+attribute for unbound methods that are implemented in C).
+
+
 .. _descriptor-invocation:
 
 Invoking Descriptors