[SPARK-29486][SQL][FOLLOWUP] Document the reason to add days field

### What changes were proposed in this pull request?
Follow up of #26134 to document the reason to add days filed and explain how do we use it

### Why are the changes needed?
only comment

### Does this PR introduce any user-facing change?
no

### How was this patch tested?
no need test

Closes #26701 from LinhongLiu/spark-29486-followup.

Authored-by: Liu,Linhong <liulinhong@baidu.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>

common/unsafe/src/main/java/org/apache/spark/unsafe/types/CalendarInterval.java

@@ -34,6 +34,10 @@ public final class CalendarInterval implements Serializable, Comparable<Calendar
   public final int days;
   public final long microseconds;
 
+  // CalendarInterval is represented by months, days and microseconds. Months and days are not
+  // units of time with a constant length (unlike hours, seconds), so they are two separated fields
+  // from microseconds. One month may be equal to 29, 30 or 31 days and one day may be equal to
+  // 23, 24 or 25 hours (daylight saving)
   public CalendarInterval(int months, int days, long microseconds) {
     this.months = months;
     this.days = days;