Improve readability and fix wording

src/python/grpcio/grpc/_cython/_cygrpc/aio/callback_common.pyx.pxi

@@ -71,7 +71,7 @@ cdef CallbackFailureHandler CQ_SHUTDOWN_FAILURE_HANDLER = CallbackFailureHandler
 
 
 class ExecuteBatchError(InternalError):
-    """Raised upon execute batch returns a fail from Core."""
+    """Raised when execute batch returns a failure from Core."""
 
 
 async def execute_batch(GrpcCallWrapper grpc_call_wrapper,
@@ -129,7 +129,7 @@ async def _receive_message(GrpcCallWrapper grpc_call_wrapper,
         # the callback (e.g. cancelled).
         #
         # Since they all indicates finish, they are better be merged.
-        _LOGGER.debug('Failed to received message from Core')
+        _LOGGER.debug('Failed to receive any message from Core')
     return receive_op.message()
 
 

src/python/grpcio/grpc/_cython/_cygrpc/aio/server.pxd.pxi

@@ -43,9 +43,9 @@ cdef class RPCState(GrpcCallWrapper):
 
 cdef class _ServicerContext:
     cdef RPCState _rpc_state
-    cdef object _loop
-    cdef object _request_deserializer
-    cdef object _response_serializer
+    cdef object _loop  # asyncio.AbstractEventLoop
+    cdef object _request_deserializer  # Callable[[bytes], Any]
+    cdef object _response_serializer  # Callable[[Any], bytes]
 
 
 cdef class _MessageReceiver: