add the comment for CheckFile function. test=develop

paddle/fluid/framework/data_feed.cc

@@ -262,10 +262,11 @@ bool MultiSlotDataFeed::CheckFile(const char* filename) {
     // It may be added '\t' character to the end of the output of reduce
     // task when processes data by Hadoop(when the output of the reduce
     // task of Hadoop has only one field, it will add a '\t' at the end
-    // of the line by default), which does not affect the correctness of
-    // the data. Therefore, it should be judged that the data is not
-    // normal when the end of each line of data contains characters
-    // which are not spaces.
+    // of the line by default, and you can use this option to avoid it:
+    // `-D mapred.textoutputformat.ignoreseparator=true`), which does
+    // not affect the correctness of the data. Therefore, it should be
+    // judged that the data is not normal when the end of each line of
+    // data contains characters which are not spaces.
     while (endptr - str != len) {
       if (!isspace(*(endptr++))) {
         VLOG(0)