### What changes were proposed in this pull request?
Fix errors and missing parts for datetime pattern document
1. The pattern we use is similar to DateTimeFormatter and SimpleDateFormat but not identical. So we shouldn't use any of them in the API docs but use a link to the doc of our own.
2. Some pattern letters are missing
3. Some pattern letters are explicitly banned - Set('A', 'c', 'e', 'n', 'N')
4. the second fraction pattern different logic for parsing and formatting
### Why are the changes needed?
fix and improve doc
### Does this PR introduce any user-facing change?
yes, new and updated doc
### How was this patch tested?
pass Jenkins
viewed locally with `jekyll serve`
Closes#27956 from yaooqinn/SPARK-31189.
Authored-by: Kent Yao <yaooqinn@hotmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
### What changes were proposed in this pull request?
The meaning of 'u' was day number of the week in SimpleDateFormat, it was changed to year in DateTimeFormatter. Now we keep the old meaning of 'u' by substituting 'u' to 'e' internally and use DateTimeFormatter to parse the pattern string. In DateTimeFormatter, the 'e' and 'c' also represents day-of-week. e.g.
```sql
select date_format(timestamp '2019-10-06', 'yyyy-MM-dd uuuu');
select date_format(timestamp '2019-10-06', 'yyyy-MM-dd uuee');
select date_format(timestamp '2019-10-06', 'yyyy-MM-dd eeee');
```
Because of the substitution, they all goes to `.... eeee` silently. The users may congitive problems of their meanings, so we should mark them as illegal pattern characters to stay the same as before.
This pr move the method `convertIncompatiblePattern` from `DatetimeUtils` to `DateTimeFormatterHelper` object, since it is quite specific for `DateTimeFormatterHelper` class.
And 'e' and 'c' char checking in this method.
Besides,`convertIncompatiblePattern` has a bug that will lose the last `'` if it ends with it, this pr fixes this too. e.g.
```sql
spark-sql> select date_format(timestamp "2019-10-06", "yyyy-MM-dd'S'");
20/03/18 11:19:45 ERROR SparkSQLDriver: Failed in [select date_format(timestamp "2019-10-06", "yyyy-MM-dd'S'")]
java.lang.IllegalArgumentException: Pattern ends with an incomplete string literal: uuuu-MM-dd'S
spark-sql> select to_timestamp("2019-10-06S", "yyyy-MM-dd'S'");
NULL
```
### Why are the changes needed?
avoid vagueness
bug fix
### Does this PR introduce any user-facing change?
no, these are not exposed yet
### How was this patch tested?
add ut
Closes#27939 from yaooqinn/SPARK-31176.
Authored-by: Kent Yao <yaooqinn@hotmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
### What changes were proposed in this pull request?
In Spark version 2.4 and earlier, datetime parsing, formatting and conversion are performed by using the hybrid calendar (Julian + Gregorian).
Since the Proleptic Gregorian calendar is de-facto calendar worldwide, as well as the chosen one in ANSI SQL standard, Spark 3.0 switches to it by using Java 8 API classes (the java.time packages that are based on ISO chronology ). The switching job is completed in SPARK-26651.
But after the switching, there are some patterns not compatible between Java 8 and Java 7, Spark needs its own definition on the patterns rather than depends on Java API.
In this PR, we achieve this by writing the document and shadow the incompatible letters. See more details in [SPARK-31030](https://issues.apache.org/jira/browse/SPARK-31030)
### Why are the changes needed?
For backward compatibility.
### Does this PR introduce any user-facing change?
No.
After we define our own datetime parsing and formatting patterns, it's same to old Spark version.
### How was this patch tested?
Existing and new added UT.
Locally document test:
Closes#27830 from xuanyuanking/SPARK-31030.
Authored-by: Yuanjian Li <xyliyuanjian@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
