Specify that Subscription#unsubscribe is idempotent

Adds SUB2b clarifying that repeated calls to `Subscription#unsubscribe`
are a no-op, matching the ably-js implementation across all three
subscription factories (LiveObject EventEmitter.off, the
PathObjectSubscriptionRegister Map.delete, and Instance which delegates
to LiveObject).

Lifted from Sachin's spec-alignment PR [1].

[1] #480

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lawrence-forooghian

specifications/features.md

@@ -1875,6 +1875,7 @@ The core SDK provides an API for wrapper SDKs to supply Ably with analytics info
 - `(SUB1)` A `Subscription` represents a registration for receiving events from a subscribe operation
 - `(SUB2)` The `Subscription` object has the following method:
   - `(SUB2a)` `unsubscribe` - deregisters the listener that was registered by the corresponding `subscribe` call. Once `unsubscribe` is called, the listener must not be called for any subsequent events
+  - `(SUB2b)` Calling `unsubscribe` more than once is a no-op
 
 ### Option types {#options}