update doc to say backprop is on gpu too now
update doc to say backprop is on gpu too now

hughperkins/DeepCL,mrgloom/DeepCL,MiniLight/DeepCL,Peilong/DeepCL,mrgloom/DeepCL,hughperkins/DeepCL,FighterLYL/DeepCL,MiniLight/DeepCL,MiniLight/DeepCL,mrgloom/DeepCL,MiniLight/DeepCL,pimms/DeepCL,FighterLYL/DeepCL,Peilong/DeepCL,Peilong/DeepCL,Peilong/DeepCL,pimms/DeepCL,hughperkins/DeepCL,FighterLYL/DeepCL,pimms/DeepCL,mrgloom/DeepCL,hughperkins/DeepCL,FighterLYL/DeepCL,mrgloom/DeepCL,FighterLYL/DeepCL,Peilong/DeepCL,MiniLight/DeepCL,pimms/DeepCL,pimms/DeepCL,hughperkins/DeepCL