invalid html (nested paragraphs) generated for function parameters

Describe the bug
The generated html contains

<li><p><strong>x</strong> – <p>hello</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>bye</p>
</div>
</p></li>

Notice the outer paragraph that comes with li, which contains inner paragraphs, that's invalid in html. I noticed this while looking at #4227 (apparently the outer p was not there in 2017). I don't know if the issue comes from sphinx or docutils (I have 0.16).

To Reproduce
a.py:

def f(y,x):
    '''
    :param y: hello
    :param x: hello

        .. note::
            bye'''
    pass

empty __init__.py, sphinx-quickstart, add .. autofunction:: a.f to index.rst and 'sphinx.ext.autodoc' to extensions in conf.py, run PYTHONPATH=. make clean html, and look at _build/html/index.html.

Expected behavior
Valid (x)html. The outer <p> doesn't look very useful to me (and the inner div and p seem redundant), but I am not an expert in html.

Environment info

• OS: Linux Debian testing x86_64
• Python version: 3.9.2
• Sphinx version: 3.4.3 (from debian) or 4.1.0 (from github)
• Sphinx extensions: sphinx.ext.autodoc

Reproduce the issue with a.py, empty init.py, index.rst, and conf.py using PYTHONPATH=. make clean html, then inspect _build/html/index.html. Compare the generated parameter markup with the expected valid HTML and determine whether the behavior comes from Sphinx or docutils. Done means the nested paragraphs are no longer emitted for this example.