EventEmitter emit spec update (#240)

* spec: Clarify EventEmitter emit behaviour when new listeners registered

* spec: Update to reflect Simon’s suggested wording

content/client-lib-development-guide/features.textile

@@ -589,6 +589,7 @@ h3(#eventemitter). EventEmitter mixin / interface
 * @(RTE4)@ @EventEmitter#once@ registers the provided listener for either the first event that is emitted when no @event@ argument is provided, or for only the first occurrence of a single named event when an @event@ argument is provided. If @once@ is called more than once with the same listener, the listener is added multiple times to its listener registry. Therefore, as an example, assuming the same listener is registered twice using @once@, and an event is emitted once, the listener would be invoked twice. However, all subsequent events emitted would not invoke the listener as @once@ ensures that each registration is only invoked once
 * @(RTE5)@ @EventEmitter#off@ deregisters a listener. If called with a specific event and a listener, it removes all registrations that match both the given listener and the given event; if called only with a listener, it removes all registrations matching the given listener, regardless of whether they are associated with an event or not; if called with no arguments, it removes all registrations, for all events and listeners
 * @(RTE6)@ @EventEmitter#emit@ emits an event, calling registered listeners with the given event name and any other given arguments. If an exception is raised in any of the listeners, the exception is caught by the @EventEmitter@ and the exception is logged to the Ably logger. Tests must exist to ensure exceptions raised in client code do not propagate and inhibit other event processing within the client library
+** @(RTE6a)@ The set of listeners called by @emit@ must not change over the course of the @emit@. That is: If a listener being called by @emit@ registers another listener, that second listener should not be called by that invocation of @emit@ (even if it would have been called had it already been present); and if a listener being called by @emit@ removes other listeners, but those other listeners would otherwise have been called during that @emit@ invocation, they should still be called. Tests should exist for both adding and removing. See "https://goo.gl/OVTtjO":https://goo.gl/OVTtjO
 
 h2(#state-conditions-and-operations). State conditions and operations