Rollup merge of #121343 - Takashiidobe:takashi/examples-for-slice, r=Mark-Simulacrum

Add examples for some methods on slices

Adds some examples to some methods on slice.

`is_empty` didn't have an example for an empty slice, even though `str` and the collections all have one, so I added that in.

`first_mut` and `last_mut` didn't have an example for what happens when the slice is empty, whereas `first` and `last` do, so I added that too.

Author: matthiaskrgr