### What changes were proposed in this pull request?
This PR modifies comment for `UTF8String.trimAll` and`sql-migration-guide.mld`.
The comment for `UTF8String.trimAll` says like as follows.
```
Trims whitespaces ({literal <=} ASCII 32) from both ends of this string.
```
Similarly, `sql-migration-guide.md` mentions about the behavior of `cast` like as follows.
```
In Spark 3.0, when casting string value to integral types(tinyint, smallint, int and bigint),
datetime types(date, timestamp and interval) and boolean type,
the leading and trailing whitespaces (<= ASCII 32) will be trimmed before converted to these type values,
for example, `cast(' 1\t' as int)` results `1`, `cast(' 1\t' as boolean)` results `true`,
`cast('2019-10-10\t as date)` results the date value `2019-10-10`.
In Spark version 2.4 and below, when casting string to integrals and booleans,
it does not trim the whitespaces from both ends; the foregoing results is `null`,
while to datetimes, only the trailing spaces (= ASCII 32) are removed.
```
But SPARK-32559 (#29375) changed the behavior and only whitespace ASCII characters will be trimmed since Spark 3.0.1.
### Why are the changes needed?
To follow the previous change.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Confirmed the document built by the following command.
```
SKIP_API=1 bundle exec jekyll build
```
Closes#33287 from sarutak/fix-utf8string-trim-issue.
Authored-by: Kousuke Saruta <sarutak@oss.nttdata.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
57a4f310df