[Docblock] Add return type to defer() helper

[tusharnain]

Summary

This pull request adds a missing @return docblock to the defer() global helper defined in Illuminate\Support\functions.php.

The helper returns either a DeferredCallback or a DeferredCallbackCollection, depending on whether a callable is passed. Currently, the docblock does not specify the return type when $callback is null, which can negatively impact IDE autocompletion and static analysis.

Benefits

• Improves IDE support (e.g., PhpStorm, VS Code)
• Enhances compatibility with static analysis tools (e.g., PHPStan, Psalm)
• Clarifies the actual behavior of the helper for future maintainers and contributors
• Does not introduce any functional changes or side effects

Why This Is Safe

• This is a docblock-only change; it does not alter runtime behavior.
• No existing code depends on the absence of the return type in the docblock.

Related

This helper is part of the newer Illuminate\Support\defer() system introduced for deferring callbacks, and this docblock ensures full developer insight into the return behavior of the helper.