update by comment

Author: Yancey0623

paddle/fluid/operators/detail/send_recv.proto

@@ -21,7 +21,7 @@ service SendRecvService {
   rpc SendVariable(VariableMessage) returns (VoidMessage) {}
   // Argument VariableMessage for GetVariable should only contain varname.
   rpc GetVariable(VariableMessage) returns (VariableMessage) {}
-  // Look up table block execution output variable name.
+  // pre-fetch variable by given variable name and Ids
   rpc PrefetchVariable(VariableMessage) returns (VariableMessage) {}
 }
 
@@ -67,7 +67,7 @@ message VariableMessage {
   bytes serialized = 8;
   // selected_rows data
   bytes rows = 9;
-  // prefetch var name
+  // Look up table block execution output variable name.
   string out_varname = 10;
 }