Doxygenate permutation_iterator.
Doxygenate permutation_iterator.

--HG--
extra : convert_revision : svn%3A83215879-3e5a-4751-8c9d-778f44bb06a5/trunk%40756

mohamed-ali/thrust,thrust/thrust,xiongzhanblake/thrust_src,thvasilo/thrust,Ricardo666666/thrust,dachziegel/thrust,raygit/thrust,thrust/thrust,xiongzhanblake/thrust_src,arnabgho/thrust,xiongzhanblake/thrust,zeryx/thrust,jaredhoberock/thrust,marksantos/thrust,jaredhoberock/thrust,Ricardo666666/thrust,mohamed-ali/thrust,sdalton1/thrust,jaredhoberock/thrust,marksantos/thrust,xiongzhanblake/thrust,jaredhoberock/thrust,GrimDerp/thrust,sarvex/thrust,thvasilo/thrust,raygit/thrust,egaburov/thrust,Ricardo666666/thrust,arnabgho/thrust,sarvex/thrust,zeryx/thrust,andrewcorrigan/thrust-multi-permutation-iterator,dachziegel/thrust,xiongzhanblake/thrust,arnabgho/thrust,jaredhoberock/thrust,mohamed-ali/thrust,sarvex/thrust,thrust/thrust,raygit/thrust,zeryx/thrust,andrewcorrigan/thrust-multi-permutation-iterator,egaburov/thrust,egaburov/thrust,zhenglaizhang/thrust,sdalton1/thrust,zhenglaizhang/thrust,thrust/thrust,dachziegel/thrust,andrewcorrigan/thrust-multi-permutation-iterator,thvasilo/thrust,GrimDerp/thrust,zhenglaizhang/thrust,xiongzhanblake/thrust_src,thrust/thrust,GrimDerp/thrust,marksantos/thrust,sdalton1/thrust