Document thrust::system::cpp::allocator
Document thrust::system::cpp::allocator

Fixes issue 442

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