Document the 'Force relay connection' switch that silently disables
ICE gathering on iOS NetBird (and similar on macOS), which was the
real cause of all peers showing Connection type: Relayed despite
STUN being correctly configured.
Also document bufferbloat diagnosis via macOS networkQuality, which
was a red herring during the P2P investigation (large RTT spikes
were the WiFi/ISP, not NetBird).
