Add safety section for SliceIndex::get_unchecked(mut) This PR adds `# Safety` sections to the documentation for the unsafe `SliceIndex::get_unchecked` and `SliceIndex::get_unchecked_mut` trait methods. The existing documentation already described the undefined behavior conditions for out-of-bounds indices and dangling slice pointers. This change keeps that content unchanged, but moves it under the standard rustdoc `# Safety` heading expected for unsafe functions. No behavior changes.