[SPARK-18508][SQL] Fix documentation error for DateDiff

## What changes were proposed in this pull request?
The previous documentation and example for DateDiff was wrong.

## How was this patch tested?
Doc only change.

Author: Reynold Xin <rxin@databricks.com>

Closes #15937 from rxin/datediff-doc.

(cherry picked from commit bce9a036)
Signed-off-by: Reynold Xin <rxin@databricks.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/datetimeExpressions.scala

@@ -1101,11 +1101,14 @@ case class TruncDate(date: Expression, format: Expression)
  * Returns the number of days from startDate to endDate.
  */
 @ExpressionDescription(
-  usage = "_FUNC_(date1, date2) - Returns the number of days between `date1` and `date2`.",
+  usage = "_FUNC_(endDate, startDate) - Returns the number of days from `startDate` to `endDate`.",
   extended = """
     Examples:
-      > SELECT _FUNC_('2009-07-30', '2009-07-31');
+      > SELECT _FUNC_('2009-07-31', '2009-07-30');
        1
+
+      > SELECT _FUNC_('2009-07-30', '2009-07-31');
+       -1
   """)
 case class DateDiff(endDate: Expression, startDate: Expression)
   extends BinaryExpression with ImplicitCastInputTypes {