document invariant of `VecDeque::as_(mut)_slice`
document invariant of `VecDeque::as_(mut)_slice`

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