docs: Documents the p2p.useStunTurn option.

config.js

         // connection.
         enabled: true,
         // The STUN servers that will be used in the peer to peer connections
+        //  useStunTurn: true, // use XEP-0215 to fetch STUN and TURN server
         stunServers: [
             { urls: "stun:stun.l.google.com:19302" },
             { urls: "stun:stun1.l.google.com:19302" },