Added comments about checking return code and the maxfd counter

docs/examples/fopen.c

@@ -153,6 +153,10 @@ fill_buffer(URL_FILE *file,int want,int waittime)
         /* get file descriptors from the transfers */
         curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);
 
+        /* In a real-world program you OF COURSE check the return code of the
+           function calls, *and* you make sure that maxfd is bigger than -1
+           so that the call to select() below makes sense! */
+
         rc = select(maxfd+1, &fdread, &fdwrite, &fdexcep, &timeout);
 
         switch(rc) {