CLEANUP: stats: fix misleading comment on top of stat_idx_info

The comment asks to update the "metrics_info" array, which does not
exist, instead it's called stat_cols_info[] and is in stats.c. Let's
mention all that to save time searching for the needed info.

While no version seems to have ever known that "metrics_info", it's not
needed to backport this as it's only a comment.

Author: wtarreau

include/haproxy/stats-t.h

@@ -253,9 +253,9 @@ enum field_scope {
 };
 
 /* Show info columns for CLI output. For any column added here, please add the
- * text representation in the metrics_info array. Please only append at the end,
- * before the ST_I_INF_MAX entry, and never insert anything in the middle
- * nor at the beginning.
+ * text representation in the stat_cols_info[] array in stats.c. Please only
+ * append at the end, before the ST_I_INF_MAX entry, and never insert anything
+ * in the middle nor at the beginning.
  */
 enum stat_idx_info {
 	ST_I_INF_NAME,