Do you have a link to the documentary?
I have been in the Marine Corps infantry and later the firearms industry for nearing 15 years. This is the first time I have heard of something like this. I have seen plenty of people shoot far more in a day through all manner of firearms including large caliber sniper rifles.
The rifle recoils against your shoulder. Your body is rigid and a lot of force is absorbed and dissipated before it moves elsewhere. If something is accelerating your shoulder fast enough to detach a retina, you are gonna have much bigger issues along everything between your retina and where the rifle contacts your shoulder.
Keep in mind that there is a lot of bullshit that gets circulated in the military just because an NCO said it 50 years ago that is just flat wrong. I remember the claim that a 50cal will rip skin off of it passes writhing 4 inches of someone. Completely wrong but repeated often by people that should know better. I would assume that the factoid you mentioned is an example of that.
tl:dr this is total bullshit.