Document P2P transport compatibility and Coturn setup

docker/coturn/.env.example

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+# Public DNS name used as the TURN authentication realm.
+TURN_REALM=
+
+# Public IPv4 address advertised by Coturn. If Coturn is behind NAT, forward
+# port 3478 (TCP and UDP) and the UDP relay range to this host.
+TURN_EXTERNAL_IP=
+
+# Static long-term credential used by the LiveSync P2P profile. Use a simple
+# username without a colon and a high-entropy password, such as hexadecimal.
+TURN_USERNAME=
+TURN_PASSWORD=