Generally on any engine, gas or diesel, it isn't a good idea to shut down if the temp is higher than normal.
The reason being, that when you shut a really hot engine down, first, the water stops circulationg, so the temp will climb even higher for a few more minutes afterward. Ever watched a mechanical temp guage after a shutdown even at normal engine temperature?
With heat, metal parts expand, and things like pistons expand at a different rate than the cylinder liners.
Along with that, you can also cook off the lubricating oil on the cylinder walls and the valve guides and stems, rendering them dry upon next start-up for a monent or two.
It may take a number of these kind of cycles, but seeing as how many of these trucks were deployed in the Middle East, I suspect that is why they had those warnings in place.
Here in VA you'd probably never have much of a worry unless you just crossed the mountains full-throttle on a 98 degree day with a full load and shut it down immediately afterward.