hello jcrown,
printing the icons in a booklet / manual with the goal to explain what things are doing or they are part of the screenshots this is not a problem.
I specified the license text now it reads like this (added the third line in the FAQ):
"Q: May I print the assets on a product box or user manual?
As long as they are part of screenshots this is also not a problem.
For a manual you are allowed to print the icons from the icon-set in order to explain what each object is doing to the player."