Add Doxygen documentation for retag and reinterpret_tag
Add Doxygen documentation for retag and reinterpret_tag

Fixes issue 456

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