What is the correct way to document a **kwargs parameter?

If you are looking for how to do this in numpydoc style, you can simply mention **kwargs in Parameters section without specifying type - as demonstrated in numpydoc example from the sphinx extension napolean and docstring guide from pandas documentation sprint 2018.

Here's an example I found from LSST developer guide which very well explains what should be the description of **kwargs parameter:

def demoFunction(namedArg, *args, flag=False, **kwargs):
    """Demonstrate documentation for additional keyword and
    positional arguments.

    Parameters
    ----------
    namedArg : `str`
        A named argument that is documented like always.
    *args : `str`
        Additional names.

        Notice how the type is singular since the user is expected to pass individual
        `str` arguments, even though the function itself sees ``args`` as an iterable
        of `str` objects).
    flag : `bool`
        A regular keyword argument.
    **kwargs
        Additional keyword arguments passed to `otherApi`.

        Usually kwargs are used to pass parameters to other functions and
        methods. If that is the case, be sure to mention (and link) the
        API or APIs that receive the keyword arguments.

        If kwargs are being used to generate a `dict`, use the description to
        document the use of the keys and the types of the values.
    """

Alternatively, building upon what @Jonas Adler suggested, I find it better to put the **kwargs and its description in Other Parameters section - even this example from matplotlib documentation guide suggests the same.

I am unable to find the actual link to the docs but this works (using Sphinx 3.4.3):

class Foo:
    """A Foo implementation

    :param str foo: Foo
    :param int bar: Bar
    :keyword str key1: kwarg 1
    :keyword str key2: kwarg 2
    :keyword int key3: kwarg 3
    """

    def __init__(self, foo, bar, **kwargs):
        pass

I think subprocess-module's docs is a good example. Give an exhaustive list of all parameters for a top/parent class. Then just refer to that list for all other occurrences of **kwargs.