Rollup merge of #39165 - frewsxcv:slice, r=GuillaumeGomez

A few improvements to the slice docs.

* Simplify `Option::iter_mut` doc example.
* Document 'empty' corner-cases for `slice::{starts_with, ends_with}`.
* Indicate 'true' as code-like.

diff --cc src/libcollections/slice.rs
Simple merge