Overload set proposal doesn't respect SFINAE

[DeadMG]

It's stated in https://github.com/asutton/wg21/blob/master/overload/fn.md that the polylambdas shown are equivalent to the overload set. That's not strictly true, as the call to an overload set can SFINAE out, whereas the call to the polylambda cannot. This doesn't make a difference if you're unambiguously calling it but in various template metaprogrammings it will make a difference. The proposal should be updated to reflect this disparity and state a stance on whether lifted sets should preserve SFINAE behaviour (they obviously should IMO).