Improve <vec::IntoIter>::get_unchecked` safety comment
Improve <vec::IntoIter>::get_unchecked` safety comment

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