Update comment about AsyncSequence

We can’t implement some of the ably#56 improvements that I hoped Swift 6
would enable, due to them requiring OS support.

Sources/AblyChat/Subscription.swift

@@ -4,7 +4,7 @@
 //
 // At some point we should define how this thing behaves when you iterate over it from multiple loops, or when you pass it around. I’m not yet sufficiently experienced with `AsyncSequence` to know what’s idiomatic. I tried the same thing out with `AsyncStream` (two tasks iterating over a single stream) and it appears that each element is delivered to precisely one consumer. But we can leave that for later. On a similar note consider whether it makes a difference whether this is a struct or a class.
 //
-// TODO: I wanted to implement this as a protocol (from which `MessageSubscription` would then inherit) but struggled to do so, hence the struct. Try again sometime. We can also revisit our implementation of `AsyncSequence` if we migrate to Swift 6, which adds primary types and typed errors to `AsyncSequence` and should make things easier; see https://github.com/ably-labs/ably-chat-swift/issues/21.
+// I wanted to implement this as a protocol (from which `MessageSubscription` would then inherit) but struggled to do so (see https://forums.swift.org/t/struggling-to-create-a-protocol-that-inherits-from-asyncsequence-with-primary-associated-type/73950 where someone suggested it’s a compiler bug), hence the struct. I was also hoping that upon switching to Swift 6 we could use AsyncSequence’s `Failure` associated type to simplify the way in which we show that the subscription is non-throwing, but it turns out this can only be done in macOS 15 etc. So I think that for now we’re stuck with things the way they are.
 public struct Subscription<Element: Sendable>: Sendable, AsyncSequence {
     private enum Mode: Sendable {
         case `default`(stream: AsyncStream<Element>, continuation: AsyncStream<Element>.Continuation)