docs: improve time.rst

[kenjis]

Description

• fix RST format
• improve explanation of toDateTimeString() toDateString() toTimeString()

Related #6116

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

[kenjis]

@datamweb Can you check the outputs (015.php to 018.php) of locale fa? No problem?
https://github.com/codeigniter4/CodeIgniter4/pull/6121/files#diff-028e477d26b38806ea1c51981970c3ae643615ba98d6eca76c7315344f8625e7

[datamweb]

@kenjis Please edit: improve explanation of toDateTimeString() toDateTimeString() ...

The output is according to what you wrote And I do not see a problem

It may be useful.
As a reminder for Iranians. Since we have our own calendar, you must use auxiliary functions to convert the date.
An example of this is JDF HELPER.
The output is as follows:

       helper(['jdf']);
       echo gregorian_to_jalali(2016,03,9,'/'); // 1394/12/19 (۱۳۹۴/۱۲/۱۹)

[kenjis]

The output is according to what you wrote And I do not see a problem

Thank you!

[MGatner]

Much clearer.