docs: add `$mailType` with `html` value for email config

[datamweb]

Description We use format html to send emails, so it's better to have this in the documentation. see https://forum.codeigniter.com/showthread.php?tid=89321

Checklist:

• [x] 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