Correctly treat built-in method (method descriptors) as methods

This fixes a bug where method descriptors were treated as data
descriptors. As the builtin_method_descriptor type is not exported
anywhere in Python, we check for the name here. As we know that
it is a descriptor, this should not be a problem.

sphinx/ext/autodoc.py

@@ -1072,7 +1072,8 @@ class AttributeDocumenter(ClassLevelDocumenter):
     @classmethod
     def can_document_member(cls, member, membername, isattr, parent):
         isdatadesc = isdescriptor(member) and not \
-                     isinstance(member, cls.method_types)
+                     isinstance(member, cls.method_types) and not \
+                     type(member).__name__ == "method_descriptor"
         return isdatadesc or \
                (isattr and not isinstance(parent, ModuleDocumenter))