added comment about host parameter format (issue 4147)

src/objective-c/ProtoRPC/ProtoRPC.h

@@ -38,6 +38,10 @@
 
 @interface ProtoRPC : GRPCCall
 
+/*
+ * host parameter should not contain the scheme (http:// or https://), only the name or IP addr
+ * and the port number, for example @"localhost:5050".
+ */
 - (instancetype)initWithHost:(NSString *)host
                       method:(ProtoMethod *)method
               requestsWriter:(GRXWriter *)requestsWriter