[FLINK-36941][docs] Update DATE_FORMAT Doc and Python Tests in 1.20

[yiyutian1]

What is the purpose of the change

To update the documentation for DATE_FORMAT, which is not accurate now. The current function behaviors are:

• DATE_FORMAT(timestamp, string) eventually calls formatTimestamp(TimestampData ts, String format, ZoneId zoneId) in DateTimeUtils.java, which uses DateTimeFormatter.

• DATE_FORMAT(string, string) eventually calls formatTimestampStringWithOffset(String dateStr, String fromFormat, String toFormat, TimeZone tz, long offsetMills) in DateTimeUtils.java, which uses SimpleDateFormat.

• Per this comment, we will fix documentation in Flink 1.20.
  https://github.com/apache/flink/pull/25828/files#r1894021871

Brief change log

• Update documentation for DATE_FORMAT.
• Add additional python tests for specific DATE_FORMAT functions.

Verifying this change

Please make sure both new and modified tests in this PR follow the conventions for tests defined in our code quality guide.

Added python tests for for specific DATE_FORMAT functions.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[flinkbot]

CI report:

• cf7bd1b Azure: SUCCESS

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