[SPARK-28471][SQL][DOC][FOLLOWUP] Fix year patterns in the comments of date-time expressions

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

@@ -647,7 +647,7 @@ case class ToUnixTimestamp(
  * Converts time string with given pattern to Unix time stamp (in seconds), returns null if fail.
  * See [https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html].
  * Note that hive Language Manual says it returns 0 if fail, but in fact it returns null.
- * If the second parameter is missing, use "yyyy-MM-dd HH:mm:ss".
+ * If the second parameter is missing, use "uuuu-MM-dd HH:mm:ss".
  * If no parameters provided, the first parameter will be current_timestamp.
  * If the first parameter is a Date or Timestamp instead of String, we will ignore the
  * second parameter.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/TimestampFormatter.scala

@@ -67,7 +67,7 @@ class Iso8601TimestampFormatter(
 }
 
 /**
- * The formatter parses/formats timestamps according to the pattern `yyyy-MM-dd HH:mm:ss.[..fff..]`
+ * The formatter parses/formats timestamps according to the pattern `uuuu-MM-dd HH:mm:ss.[..fff..]`
  * where `[..fff..]` is a fraction of second up to microsecond resolution. The formatter does not
  * output trailing zeros in the fraction. For example, the timestamp `2019-03-05 15:00:01.123400` is
  * formatted as the string `2019-03-05 15:00:01.1234`.

sql/core/src/main/scala/org/apache/spark/sql/functions.scala

@@ -2634,8 +2634,8 @@ object functions {
    * See [[java.time.format.DateTimeFormatter]] for valid date and time format patterns
    *
    * @param dateExpr A date, timestamp or string. If a string, the data must be in a format that
-   *                 can be cast to a timestamp, such as `yyyy-MM-dd` or `yyyy-MM-dd HH:mm:ss.SSSS`
-   * @param format A pattern `dd.MM.yyyy` would return a string like `18.03.1993`
+   *                 can be cast to a timestamp, such as `uuuu-MM-dd` or `uuuu-MM-dd HH:mm:ss.SSSS`
+   * @param format A pattern `dd.MM.uuuu` would return a string like `18.03.1993`
    * @return A string, or null if `dateExpr` was a string that could not be cast to a timestamp
    * @note Use specialized functions like [[year]] whenever possible as they benefit from a
    * specialized implementation.