Document our reliance on the side effect

[maratal]

I've put a few thoughts here. Also, when I first looked at this PR, I was confused about why the ARTSRWebSocket class, which also makes use of +[NSRunLoop ARTSR_networkRunLoop], hadn't been modified to wait for the run loop to become available. I think it's because ARTSRWebSocket makes use of the -[ARTSRProxyConnect openNetworkStreamWithCompletion:] method, which makes sure that the run loop is set up before it calls its completion handler. It would be good to document our reliance on this side effect.

Originally posted by @lawrence-forooghian in https://github.com/ably/ably-cocoa/pull/1538#pullrequestreview-1197960432

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The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3116