Document Repeat::last panic behavior

[majiayu000]

Fixes #149707

Documents that Repeat::last always panics because Repeat is infinite.

Tests:

• ./x fmt --check
• ./x doc library/core
• ./x test library/core

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: libs
• libs expanded to 8 candidates
• Random selection from LawnGnome, Mark-Simulacrum, jhpratt