Added documentation for connection and label properties in Protocol class

sjrpc/core/protocols/__init__.py

@@ -34,10 +34,18 @@ class Protocol(object):
 
     @property
     def connection(self):
+        '''
+        The :class:`~sjrpc.core.RpcConnection` instance which handle
+        this protocol.
+        '''
         return self._connection
 
     @property
     def label(self):
+        '''
+        The label binded to this protocol in the
+        :class:`~sjrpc.core.RpcConnection` instance.
+        '''
         return self._label
 
     def send(self, payload):