Just learned about this. Quite sure this warning was not part of instructions for the 4 Aisin pumps I have installed. Can't believe a couple dry turns by hand would be an issue but starting an engine without coolant obviously is a bad idea.
http://www.be.aisin-europe.com/files/7614/2425/2384/Water_pump.pdf
http://www.be.aisin-europe.com/files/7614/2425/2384/Water_pump.pdf