docs: improve banning users

[kenjis]

Description See https://forum.codeigniter.com/showthread.php?tid=90733

Checklist:

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