Improve documentation of slice::get_unchecked() / split_at_unchecked()
Improve documentation of slice::get_unchecked() / split_at_unchecked()

Thanks to Ivan Tham, who gave the majority of these suggestions during
their review.

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