Re: [webrtc-pc] Add paragraph about RtpContributingSources being updated simultaneously.

@taylor-b Ah, I had forgotten about the performance issue @jesup and I were supposed to look at here. In light of that, I'm fine either way.

Your implementation looks good to me. As for indistinguishable, you might consider queuing a task instead of a micro-task, otherwise I could observe a difference like this:
```js
let a = receiver.getContributingSources();
await Promise.resolve();
let b = receiver.getContributingSources();
log(JSON.stringify(a) == JSON.stringify(b)); // false
```
Not a problem per se, just different. I'm actually not sure what the TAG view is on state changes in the microtask queue.

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