Probably found in all the service manuals with a turbo just to cover themselves. Like others have mentioned this was common practice with turbo's through the 80's and even in the 90's it was common knowledge. I recall most of my friends installing "turbo timers" as well, basically when you turn the keys off the engine would stay on for a timed amount of time to let the oil and everything to continue to run and circulate and then shut off so you wouldn't have to sit there inside and wait. I'm sure lots of owners of turbo's in the 90's didn't follow this, the cars still survived but it wasn't good for the car, especially after hard driving.