Rollup merge of rust-lang#119998 - rustbot:docs-update, r=ehuss

Update books

## rust-lang/nomicon

1 commits in f6bd083c4ccfc4ce6699b8b4154e3c45c5a27a8c..6bc2415218d4dd0cb01433d8320f5ccf79c343a1
2024-01-03 04:01:04 UTC to 2024-01-03 04:01:04 UTC

- Update an example of `thread_local` to use `local_key_cell_methods` (rust-lang/nomicon#438)

## rust-lang/reference

1 commits in 3565c7978cfc9662f5963b135690ff9cbbfa0318..8c77e8be9da1a9c70545556218d563c8d061f1fd
2024-01-08 15:16:51 UTC to 2024-01-08 15:16:51 UTC

- Document that <- is a single token (rust-lang/reference#1424)

## rust-lang/rust-by-example

4 commits in c0be6299e52e4164c30ba6f41bd0ad0aaee64972..ddf5cb0e6ee54ba2dd84c8ca3e1314120014e20d
2024-01-13 11:06:22 UTC to 2024-01-13 11:01:00 UTC

- Update attribute.md, (rust-lang/rust-by-example#1791)
- fix: typo `unusude` (rust-lang/rust-by-example#1797)
- Update print.md (rust-lang/rust-by-example#1796)
- Update alias.md (rust-lang/rust-by-example#1790)

## rust-lang/rustc-dev-guide

4 commits in d13e85152a977cd0bcaf583cf5f49e86225697de..4af29d1a7f64f88a36539662c6a84fe1fbe6cde1
2024-01-14 10:42:53 UTC to 2024-01-06 17:47:01 UTC

- Expand upon PR guidelines (rust-lang/rustc-dev-guide#1851)
- Correct the link to rust reference (rust-lang/rustc-dev-guide#1848)
- Explain the important concepts of exhaustiveness checking (rust-lang/rustc-dev-guide#1830)
- Add guide for rustdoc search implementation (rust-lang/rustc-dev-guide#1846)