Partial functions are not documented when the original function comes from a different module

Describe the bug When using autodoc, partial functions do not appear in the HTML output if the original function (used to create the new partial function) comes from a different module.

To Reproduce Steps to reproduce the behavior:

$ git clone https://github.com/scganterh/sphinx_bug_partial_functions.git
$ cd sphinx_bug_partial_functions
$ pip install sphinx
$ make html
$ # open html/b.html and see that no documentation for the `b` function is generated.

Expected behavior I expect to see the documentation for b.b generated the same way as the documentation for b.d.

Your project https://github.com/scganterh/sphinx_bug_partial_functions

Screenshots You can see there is no entry for b.b.

Environment info

• OS: Linux
• Python version: 3.6.2
• Sphinx version: 3.3.1
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools: Browser