Google has added a handy reference sheet for all available attributes, not previously available in simple list form in the dashboard before.
The link to download the attribute spreadsheet is available from the Import Locations screen, seen in the screen capture below:
The reference sheet is simply a comprehensive list of all available attributes and does not indicate which attributes are accessible per category selection. Until Google makes it clear which attributes are associated to each category, the attribute selection process might be a bit of trial and error. Despite this shortcoming, it is a great start and a step in the right direction to helping business owners choose the right categories to define their businesses, and select attributes to tell users more about the location.