I could see where some specific movements and poses might be bad, but cutting out all yoga all together? That sounds honestly absurd to me. I hope you can get a second opinion from another doctor, and regardless talk to someone - maybe a physical therapist? - who can give you a realistic idea of which MOTIONS might be a bad idea. There's a fair amount of yoga that doesn't have to stress your knees at all, and if your doctor doesn't even consider that, he doesn't sound very useful to me.