Doxygenate permutation_iterator.
Doxygenate permutation_iterator.

levendlee/thrust,google-code-export/thrust,rdmenezes/thrust,lishi0927/thrust,malenie/thrust,rdmenezes/thrust,rdmenezes/thrust,h1arshad/thrust,bfurtaw/thrust,levendlee/thrust,Vishwa07/thrust,UIKit0/thrust,levendlee/thrust,h1arshad/thrust,UIKit0/thrust,malenie/thrust,lishi0927/thrust,Vishwa07/thrust,allendaicool/thrust,Vishwa07/thrust,julianromera/thrust,allendaicool/thrust,allendaicool/thrust,lishi0927/thrust,h1arshad/thrust,levendlee/thrust,rdmenezes/thrust,bfurtaw/thrust,hemmingway/thrust,hemmingway/thrust,bfurtaw/thrust,malenie/thrust,hemmingway/thrust,UIKit0/thrust,Vishwa07/thrust,UIKit0/thrust,julianromera/thrust,google-code-export/thrust,julianromera/thrust,hemmingway/thrust,h1arshad/thrust,julianromera/thrust,bfurtaw/thrust,allendaicool/thrust,malenie/thrust,google-code-export/thrust,google-code-export/thrust,lishi0927/thrust