It depends on your company. Mine had nor formal documents, so I wrote up my own letter explaining when I would be back, how I was using vacation time, that I expected my medical benefits to continue to be paid while I was gone. I also noted for them that I would give them my child's information to add to insurance once each of them were born and reminded them when I returned I would need a private place to pump as I would be breastfeeding.
You'll have disability forms to fill out as well, but those you get from your doctor or the website. Once you fill those out, they go back to your doctor, not your HR folks.
Good luck...it's a pain navigating your way through it, but thank God for the internet!!!