Add document for custom protocol

sanic/sanic.py

@@ -251,7 +251,7 @@ class Sanic:
         :param workers: Number of processes
         received before it is respected
         :param loop: asyncio compatible event loop
-        :param protocol: Subclass of asyncio.Protocol
+        :param protocol: Subclass of asyncio protocol class
         :return: Nothing
         """
         self.error_handler.debug = True

sanic/server.py

@@ -245,7 +245,7 @@ def serve(host, port, request_handler, error_handler, before_start=None,
     :param request_max_size: size in bytes, `None` for no limit
     :param reuse_port: `True` for multiple workers
     :param loop: asyncio compatible event loop
-    :param protocol: Subclass of asyncio.Protocol
+    :param protocol: Subclass of asyncio protocol class
     :return: Nothing
     """
     loop = loop or async_loop.new_event_loop()