B018 complains about f-strings as docstrings... and it's right! (but a more specific message would be helpful)

[Zac-HD]

CONSTANT = "magic value"

def f():
    f"""Return {CONSTANT!r}."""  # warning B018
    return CONSTANT

assert f.__doc__ is not None  # fails!

I was originally reporting this as a bug, but it turns out that f-strings don't work as docstrings! IMO this deserves to be split out into a new B019 check with a more specific message.

[cooperlees]

Nice. Would totally accept this PR if we can cleanly split this error out.

First, we need to workout our long term design with string detection in general for B018 as #208 has identified from the abstract use case sphinx uses and I am sure other edge cases will come up.