Document iterator_facade
Document iterator_facade

Fixes #309

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