audiobridge doc update - pin added to changeroom doc (#3389)

Added documentation for pin attribute in AudioBridge's changedroom

src/plugins/janus_audiobridge.c

@@ -1033,6 +1033,7 @@ room-<unique room ID>: {
  "request" : "changeroom",
  "room" : <numeric ID of the room to move to>,
  "id" : <unique ID to assign to the participant; optional, assigned by the plugin if missing>,
+ "pin" : "<password required to join the room, optional>",
  "group" : "<group to assign to this participant (for forwarding purposes only; optional, mandatory if enabled in the new room)>",
  "display" : "<display name to have in the room; optional>",
  "token" : "<invitation token, in case the new room has an ACL; optional>",