RTP stats lifetime: clarify "when RTP sender is configured"

[henbos]

This is arguably(?) editorial or at least fixing a mistake, but I'd like to tweak this sentence:

The RTCOutboundRtpStreamStats is created when the RTP sender is configured by setting a local or remote SDP answer (via setLocalDescription() or setRemoteDescription() and the signaling state returning to "stable").

Because it seems to neglect "pr-answer" which can also configure the RTP sender for sending but without returning to "stable". I think we should instead change this sentence to say that "by setting local or remote SDP and currentDirection becomes non-null" since that is how I implemented this.

[henbos]

Credit to @fippo for the "has current_direction" way of implementing this, it seems the most straightforward so let's make spec say it explicitly