PHPDoc: use @return $this for fluent methods

[caugner]

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

• Some @return annotations used wrong casing (e.g. @return CSV instead of @return Csv)
• Most classes can be extended and therefore the annotation @return static would be more appropriate to avoid inadvertent type narrowing.
• The @return $this annotation is more explicit than @return static.

[caugner]

@PowerKiKi Any chance to get this merged? This fixes errors reported by Psalm like this:

ERROR: InvalidClass - src/.../ExcelExport.php:135:17 - Class or interface PhpOffice\PhpSpreadsheet\Writer\CSV has wrong casing
                $writer->setDelimiter(',')