[docs] Comment connected socket availability for adapters (socketio#2081)

Author: kapouer

lib/socket.js

@@ -283,6 +283,8 @@ Socket.prototype.leaveAll = function(){
 /**
  * Called by `Namespace` upon successful
  * middleware execution (ie: authorization).
+ * Socket is added to namespace array before
+ * call to join, so adapters can access it.
  *
  * @api private
  */