Improve documentation for slice swap/copy/clone operations.
Improve documentation for slice swap/copy/clone operations.

Fixes #45636.

- Demonstrate how to use these operations with slices of differing
  lengths
- Demonstrate how to swap/copy/clone sub-slices of a slice using
  `split_at_mut`

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