Allow transitions in docstrings

Is your feature request related to a problem? Please describe.
I am trying to use transitions (----) in the docstrings of my Python package members but Sphinx outputs an error message when I do that and the generated html does not contain a transition.

Here is a minimal example. The function below belongs to my_module.

def foobar():
    r"""Foo

    ----

    Bar
    """
    pass

This is my Sphinx's main index.rst file:

.. autosummary::
    :toctree: _api_members

    my_module.foobar

Trying to make the html will output the following message:

docstring of my_module.foobar:3: WARNING: Unexpected section title or transition.

And as can be seen below, the output does not contain the expected transition.

I believe the only changes I did to my default conf.py are:

extensions = [
    'sphinx.ext.autodoc',
    'sphinx.ext.autosummary',
]

autosummary_generate = True
autosummary_imported_members = True

Describe the solution you'd like
I would like to be able to use ---- in docstrings similarly to regular rst.

Describe alternatives you've considered
The only way I found to solve this is to use raw html to print a <hr>, but this is really inelegant solution that polutes the docstrings.

def foobar():
    r"""Foo

    .. raw:: html

        <hr>

    Bar
    """
    pass

Producing the expected output with the horizontal rule:

Start with the minimal docstring example and the conf.py settings for autodoc and autosummary, then build the index.rst example to reproduce the warning. Trace the docstring rendering path and verify that the build no longer reports an unexpected transition and that the generated HTML contains the transition.