Fix docblock for return values

To make PhpStorm happier, and I assume other editors, I have cleaned up the @return docblock for the Router class.

You will also notice I removed the `return` attribute on the following methods:

- `prefixedResource()`
- `addResourceUpdate()`
- `model()`

This is because the methods called after the return attribute return void. Let me know if you wish for me to revert the `return` attribute and send a PR with only PhpDoc changes.

Cheers,
Andrew