docstring preview prevented by leading `type: ...` comments for PEP-484 in py2

[neilvyas]

This definition won't have its docstring previewed:

def f(a):
    # type: (int) -> int
    """Bars the foo.
    """
    return a + 3

PEP-484, specifically the section for syntax in py2.7.x.

I might take a stab at this later if anyone else considers it desirable.