Add documentation about the UB of the stack2reg optimization
Add documentation about the UB of the stack2reg optimization

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust