#1992 - Improve examples for thumb/resize/file path/url functions in item model

Documentation-only changes to examples used in:
- Item_Model::url
- Item_Model::abs_url
- Item_Model::file_path
- Item_Model::file_url
- Item_Model::resize_path
- Item_Model::resize_url
- Item_Model::thumb_path
- Item_Model::thumb_url