Add note to `Vec::split_at_spare_mut` docs that the method is low-level
Add note to `Vec::split_at_spare_mut` docs that the method is low-level

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