Google convention doesn't raise D417 when section Returns is present

[freddyaboulton]

Pydocstyle won't raise D417 when a section called Returns is present in the docstring.

With pydocstyle 5.0.1, the following example won't raise a D417 (pydocstyle example.py --convention google):

"""Example."""

def foo(a, b, c):
    """Docstring.

    Arguments:
        a (int): First number
        b (int): Second number
        d (int): Third number

    Returns:
       int: sum
    """
    return a + b + c

When I change Returns to Returns, D417 is raised. Also, when no Returns or Return section is present, D417 is raised.

[samj1912]

Copy of #459