More comments on the local echo

lib/crypto/index.js

@@ -797,6 +797,9 @@ Crypto.prototype.encryptEventIfNeeded = function(event, room) {
         return null;
     }
 
+    // We can claim and prove ownership of all our device keys in the local
+    // echo of the event since we know that all the local echos come from
+    // this device.
     var myKeys = {
         curve25519: this._olmDevice.deviceCurve25519Key,
         ed25519: this._olmDevice.deviceEd25519Key,