As of the 2021 release, my original solution does not work. 

The new solution is to:

1. Go to Sch OI, right-click in the "Tax treaty article" field and then select "About Line L1(b)" at the bottom of the pop-up menu.

2. This selection will link to the following Help Center entry:

3. Click on the blue "applicable tax treaty article" link and the following list will open.  I suggest that you save/print this list for future reference: