Update description

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

@@ -473,11 +473,19 @@ case class Lag(input: Expression, offset: Expression, default: Expression)
 /**
  * The NthValue function returns the value of `input` at the row that is the `offset`th row of
  * the window frame (counting from 1). Offsets start at 0, which is the current row. When the
- * value of `input` is null at the `offset`th row, null is returned.
+ * value of `input` is null at the `offset`th row or there is no such an `offset`th row, null
+ * is returned.
  *
  * @param input expression to evaluate `offset`th row of the window frame.
  * @param offset rows to jump ahead in the partition.
  */
+@ExpressionDescription(
+  usage = """
+    _FUNC_(input[, offset]) - Returns the value of `input` at the row that is the`offset`th row
+      of the window frame (counting from 1). If the value of `input` at the `offset`th row is
+      null, null is returned. If there is no such an offset row (e.g., when the offset is 10,
+      size of the window frame less than 10), null is returned.
+  """)
 case class NthValue(input: Expression, offset: Expression)
     extends OffsetWindowFunction {