Add a warning to `signal` of `AnyProperty`.
Add a warning to `signal` of `AnyProperty`.

ikesyo/ReactiveCocoa,sandyway/ReactiveCocoa,walkingsmarts/ReactiveCocoa,andersio/ReactiveCocoa,ohwutup/ReactiveCocoa,ikesyo/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,ReactiveCocoa/ReactiveSwift,sandyway/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,calebd/ReactiveCocoa,calebd/ReactiveCocoa,victorlin/ReactiveCocoa,andersio/ReactiveCocoa,ohwutup/ReactiveCocoa,walkingsmarts/ReactiveCocoa,ohwutup/ReactiveCocoa,walkingsmarts/ReactiveCocoa,victorlin/ReactiveCocoa,calebd/ReactiveCocoa,sandyway/ReactiveCocoa,andersio/ReactiveCocoa,victorlin/ReactiveCocoa