Add return types to PHPDoc comments for Chullo

[dannylamb]

Functions in https://github.com/Islandora-CLAW/chullo/blob/master/src/IFedoraApi.php return a \Psr\Http\Message\ResponseInterface, but it isn't noted in the PHPDoc comments. Add the @return annotation where appropriate.

This ticket is low-priority, but also low-hanging. Adding these annotations will enhance the auto-complete experience when using IDE's, so there's a tangible benefit to this.