It might be clearer to remove the "Upon subscription" part of this first sentence, and clarify that this is referring to the `+createSignal:` constructor. Something like:

> If a `RACSignal` is created using `+createSignal:`, the given block will be called once for each new subscription.

Also, instead of backticks, `RACSignal` can be written as `[RACSignal][]` to use the link in the footnotes.