fix: document PostToPost::to as an array

[mindctrl]

Description of the Change

Changes the PHPDoc on PostToPost::to from string to array, so that it accurately reflects the data stored there, which fixes warnings where an IDE thinks things like count() are being run against a string when they're not.

Benefits

Better developer experience.

Possible Drawbacks

None that I'm aware of.

Verification Process

• I changed the PHPDoc from string to array and verified that my IDE no longer complains about count() being used with an improper data type.

No functional changes.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Fixed PHPDoc for PostToPost::to property.