docs: add note to Time::difference() and DST

[kenjis]

Description
See https://forum.codeigniter.com/showthread.php?tid=90472

This may be a bug, but I don't know how to fix.

        $current = Time::parse('2024-03-31', 'Europe/Madrid');
        $test    = Time::parse('2024-04-01', 'Europe/Madrid');

        $diff = $current->difference($test);

        echo $diff->getDays(); // 0

        $current = new DateTimeImmutable('2024-03-31', new DateTimeZone('Europe/Madrid'));
        $test    = new DateTimeImmutable('2024-04-01', new DateTimeZone('Europe/Madrid'));

        $diff = $current->diff($test);

        echo $diff->format("%a"); // 1

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[datamweb]

This is not related to this PR, but I recently read the documentation for Time::class.
The point is that dates in the database are commonly stored as yyyy-MM-dd HH:mm:ss, but there is no example of using this form in the document. I think you can use 2024-03-31 00:00:00 instead of 2024-03-31 .

[kenjis]

I sent #8668

[kenjis]

I sent another PR to fix the behavior: #8659

[kenjis]

Go to #8661