Shorter section underlines trigger ambiguous validation

[HealthyPear]

While testing #454 I noticed that for the following docstring,

    """Bad, bad docstring!

    Parameters
    ---------
    x : `int`
    y:
        Another variable.

    Returns
    -------
    z : `int`
        The sum of ``x`` and ``y``.
    """

which contains many issues, but the issue of the shorter underline triggers a rather misterious
PR01 | Parameters {'x', 'y'} not documented.

In fact, after adding the missing dash I get the next issues, PR01, PR02, PR07 and PR10.

From the list of available validation checks the shorter underline problem is not present, I guess because it's more an RsT issue, but since numpydoc uses RsT (unless I overlooked this), I think it should be a proper validation check.