Callables implementing descriptor protocol are documented as attributes

Describe the bug

A callable that implements the descriptor protocol will be documented by autosummary as an attribute rather than a function. e.g.

class _MyFunction:
    def __call__(self):
        """Example function-like object"""
        print("Hello, world")

    def __get__(self, instance, owner):
        from functools import partial
        return partial(self.__call__, instance)

To Reproduce
From the root of the included project,

$ make -C docs html

docs/source/generated/autobugdemo.func.rst now contains the autosummary for the callable function, which is using autoattribute.

Expected behavior

I expect autofunction to be generated since it is a callable object.

Your project
sphinx-autofunc-bug.zip

Environment info

• OS: Ubuntu Linux
• Python version: 3.7.1
• Sphinx version: 2.1.2
• Sphinx extensions: sphinx.ext.autodoc, sphinx.ext.autosummary

Additional context
I am creating a function decorator that wraps the function in a callable object. This object must implement the descriptor protocol in order to correctly act like a method.

Sphinx does have checks for built in Function-like types (e.g. WrapperDescriptorType), but as far as I can tell there's no equivalent for user types. I think the relevant special casing is in isattributedescriptor

https://github.com/sphinx-doc/sphinx/blob/5d89c30dab810117421b6262362b1b45432c9f8e/sphinx/util/inspect.py#L171-L195

Start with sphinx/util/inspect.py, particularly isattributedescriptor, which the issue identifies as relevant to descriptor classification. Reproduce the behavior from the supplied project with make -C docs html and inspect docs/source/generated/autobugdemo.func.rst. Done means the callable descriptor is documented with autofunction rather than autoattribute.