Rollup merge of rust-lang#85205 - ehuss:update-books, r=ehuss

Update books

## nomicon

4 commits in 8551afbb2ca6f5ea37fe58380318b209785e4e02..55de6fa3c1f331774da19472c9ee57d2ae9eb039
2021-04-01 21:58:50 +0900 to 2021-05-12 00:31:01 +0900
- Clarify some of the language around marking traits safe/unsafe. (rust-lang/nomicon#268)
- Use pointer 'add' instead of 'offset' (rust-lang/nomicon#265)
- Adjust Vec to build on stable Rust (rust-lang/nomicon#223)
- Update link to c++ atomic ordering docs (rust-lang/nomicon#264)

## reference

3 commits in d23f9da8469617e6c81121d9fd123443df70595d..5aa457bf1b54bd2cd5d4cf49797f29299bdf89a7
2021-04-28 11:16:44 -0700 to 2021-05-05 08:39:22 -0700
- Explicitly state result of compound assignment (rust-lang/reference#1013)
- Adjust the definition of `target_family` (rust-lang/reference#1006)
- Fix typo in `Traits` (rust-lang/reference#1012)

## book

2 commits in 50dd06cb71beb27fdc0eebade5509cdcc1f821ed..55a26488ddefc8433e73a2e8352d70f7a5c7fc2b
2021-04-23 13:21:54 -0500 to 2021-05-09 12:03:18 -0500
- Past-tensify "lead" -> "led" (rust-lang/book#2717)
- Merge pull request rust-lang/book#2718 from rust-lang/update-rustc

## rust-by-example

2 commits in e0a721f5202e6d9bec0aff99f10e44480c0da9e7..5f8c6da200ada77760a2fe1096938ef58151c9a6
2021-04-27 09:32:15 -0300 to 2021-04-29 08:08:01 -0300
- Fix Typo in LRBE section; closes rust-lang/rust-by-example#1434 (rust-lang/rust-by-example#1437)
- Add some tests to cargo/test.md. Partially addresses rust-lang/rust-by-example#1304 (rust-lang/rust-by-example#1438)

## rustc-dev-guide

3 commits in e72b43a64925ce053dc7830e21c1a57ba00499bd..1e6c7fbda4c45e85adf63ff3f82fa9c870b1447f
2021-04-27 12:35:37 -0700 to 2021-05-10 13:38:24 +0900
- Unified CPU Requirements (rust-lang/rustc-dev-guide#1126)
- add 'waiting-for-review' incantation to main contrib page (rust-lang/rustc-dev-guide#1124)
- Link to Zulip search for finding the most recent check-in (rust-lang/rustc-dev-guide#1118)