The documentation doesn't tell much about the possible values. Searching the GitHub repos, I found a few occurrences https://aps.autodesk.com/en/docs/viewer/v2/reference/javascript/button/#seticon-iconclass , but nothing tells more about the values.
I would have opened a GitHub issue about extending the documentation, but I couldn't find the repository for the viewer.
This article can help you to get the Icons provided by the viewer stylesheet.
Then if you want too add custom icon this second article can be useful: Use custom icons on the Viewer toolbar.
Also, please note that the link you provided to the viewer documentation refers to the v2 and we are now in v7 !