[SPARK-45070][SQL][DOCS] Describe the binary and datetime formats of to_char/to_varchar

[MaxGekk]

What changes were proposed in this pull request?

In the PR, I propose to document the recent changes related to the format of the to_char/to_varchar functions:

1. binary formats added by [SPARK-44983][SQL] Convert binary to string by to_char for the formats: hex, base64, utf-8 #42632
2. datetime formats introduced by [SPARK-44868][SQL] Convert datetime to string by to_char/to_varchar #42534

Why are the changes needed?

To inform users about recent changes.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

By CI.

Was this patch authored or co-authored using generative AI tooling?

No.

[dongjoon-hyun]

+1, LGTM.

[dongjoon-hyun]

To @MaxGekk , it seems our Linter CI doesn't allow this kind of human-friendly formatting in the comment.

• https://github.com/MaxGekk/spark/actions/runs/6076537960/job/16484751098

The scalafmt check failed on connector/connect at following occurrences:
Requires formatting: functions.scala
Before submitting your change, please make sure to format your code using the following command:
./build/mvn -Pscala-2.12 scalafmt:format -Dscalafmt.skip=false -Dscalafmt.validateOnly=false -Dscalafmt.changedOnly=false -pl connector/connect/common -pl connector/connect/server -pl connector/connect/client/jvm
Error: Process completed with exit code 1.

The recommended style is something like the following. Please fix the style by run the above command line.

- * brackets if the input value is negative.</li> </ul>
- * If `e` is a datetime, `format` shall be a valid datetime pattern, see
- * <a href="https://spark.apache.org/docs/latest/sql-ref-datetime-pattern.html">Datetime Patterns</a>.
- * If `e` is a binary, it is converted to a string in one of the formats:
- * <ul>
- * <li>'base64': a base 64 string.</li>
- * <li>'hex': a string in the hexadecimal format.</li>
- * <li>'utf-8': the input binary is decoded to UTF-8 string.</li>
- * </ul>
+ * brackets if the input value is negative.</li> </ul> If `e` is a datetime, `format` shall be
+ * a valid datetime pattern, see <a
+ * href="https://spark.apache.org/docs/latest/sql-ref-datetime-pattern.html">Datetime
+ * Patterns</a>. If `e` is a binary, it is converted to a string in one of the formats: <ul>
+ * <li>'base64': a base 64 string.</li> <li>'hex': a string in the hexadecimal format.</li>
+ * <li>'utf-8': the input binary is decoded to UTF-8 string.</li> </ul>

[MaxGekk]

Thank you, @dongjoon-hyun

[MaxGekk]

Merging to master. Thank you, @dongjoon-hyun and @cloud-fan for review.