autodoc mock imports results in undocumented object

[PhilippSelenium]

Describe the bug

When mocking an import with autodoc_mock_imports objects that derive from the mocked package have no docstring. See picture below where the foo.FOO constant is not documented.

How to Reproduce

foo.py

"""Foo module."""

from asdf import qwerty

FOO = qwerty.FOO
"""Foo constant."""

ASDF = 1
"""ASDF."""

def foo():
    """Return foo :py:const:`foo.FOO` :py:const:`foo.ASDF`."""
    return FOO

conf.py

import os
import sys
sys.path.insert(0, os.path.abspath(os.pardir))

project = 'sphinx'
copyright = '2021, sphinx'
author = 'sphinx'

extensions = ['sphinx.ext.autodoc']
templates_path = ['_templates']
exclude_patterns = []
html_theme = 'alabaster'
html_static_path = ['_static']

autodoc_mock_imports = ['asdf']

run:

make html SPHINXOPTS=-Ea

Expected behavior

The docstring of foo.FOO is displayed in the documentation

Your project

See How to reproduce

Screenshots

OS

Linux

Python version

3.8.5

Sphinx version

4.3.0

Sphinx extensions

No response

Extra tools

No response

Additional context

No response