docs: fix Sphinx errors

[kenjis]

Description

• fix Sphinx errors

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

[MGatner]

I'm not that familiar with Sphinx and syntax but I trust you to proceed if these changes are all fixes.

[MGatner]

If you want a better review I think @paulbalandan has some Sphinx experience.

[kenjis]

Now "Test User Guide" check fails. See #6368
https://github.com/codeigniter4/CodeIgniter4/runs/7784160957?check_suite_focus=true
This PR fixes the errors.

I also think the way in this PR is the correct way to describe FQCN.

[kenjis]

It looks good!
https://codeigniter4.github.io/CodeIgniter4/libraries/email.html#CodeIgniter\Email\Email
https://codeigniter4.github.io/CodeIgniter4/database/query_builder.html#CodeIgniter\Database\BaseBuilder

Compare:
https://codeigniter4.github.io/userguide/libraries/email.html#class-reference
https://codeigniter4.github.io/userguide/database/query_builder.html#class-reference
Cannot link to the class.