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Rollup merge of #157323 - majiayu000:docs-repeat-last-panic, r=jhpratt
Document Repeat::last panic behavior
Fixes rust-lang/rust#149707
Documents that `Repeat::last` always panics because `Repeat` is infinite.
Tests:
- `./x fmt --check`
- `./x doc library/core`
- `./x test library/core`
