Add return type to hook_file_download implementations

Problem/Motivation

See #3483037: [META] Add return types to hook implementations

This is the last group of hooks in Hook classes to be updated.

Steps to reproduce

grep "fileDownload\\\\.* has no return" core/.phpstan-baseline.php

Proposed resolution

Add array|int|null to the hook documentation. Add the same to hook implementations, or a narrower set where possible.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet