Rollup merge of #84191 - ehuss:update-books, r=ehuss

Update books

## nomicon

1 commits in 6fe476943afd53a9a6e91f38a6ea7bb48811d8ff..8551afbb2ca6f5ea37fe58380318b209785e4e02
2021-03-10 07:28:57 +0900 to 2021-04-01 21:58:50 +0900
- Add example of thinking about Send/Sync's soundness (rust-lang/nomicon#259)

## reference

10 commits in fd97729e2d82f8b08d68a31c9bfdf0c37a7fd542..e1abb17cd94cd5a8a374b48e1bc8134a2208ed48
2021-03-28 14:29:19 -0700 to 2021-04-07 08:09:48 -0700
- Update introduction.md (rust-lang/reference#1004)
- clarify UB for raw ptr deref (rust-lang/reference#1000)
- Update lint level documentation. (rust-lang/reference#998)
- Add rustdoc to tool lints. (rust-lang/reference#997)
- Link to ptr::addr_of on raw pointer docs (rust-lang/reference#993)
- apply rust-lang/reference#950 to STYLE.md (rust-lang/reference#980)
- Tuple Passover rust-lang/reference#2 (rust-lang/reference#990)
- Fix typo in macros-by-example.md (rust-lang/reference#996)
- Clarify object safety rules for methods striked from the vtable (rust-lang/reference#965)
- Add const generic args to const contexts. (rust-lang/reference#995)

## rust-by-example

1 commits in 29d91f591c90dd18fdca6d23f1a9caf9c139d0d7..c80f0b09fc15b9251825343be910c08531938ab2
2021-03-23 09:03:39 -0300 to 2021-04-08 10:28:17 -0300
- fix compile bug with panic! (rust-lang/rust-by-example#1433)

## rustc-dev-guide

11 commits in 0687daa..a9bd2bb
2021-03-28 13:33:56 -0400 to 2021-04-09 18:12:21 -0400
- Improve formatting and update info in "method lookup" section
- Change wording a bit: `module` => `crate`
- fix typo (rust-lang/rustc-dev-guide#1107)
- fix typo
- Mention CI build of LLVM in build instruction
- Fix rustdocs test command typo (rust-lang/rustc-dev-guide#1103)
- Update the "LLVM updates" section
- Fix a link about Rustdoc internals
- Add quickstart for adding a new optimization (rust-lang/rustc-dev-guide#1094)
- Add back example of {{cwd}} (rust-lang/rustc-dev-guide#1099)
- Document test input normalization

## embedded-book

1 commits in d3f2ace94d51610cf3e3c265705bb8416d37f8e4..569c3391f5c0cc43433bc77831d17f8ff4d76602
2021-03-17 07:53:09 +0000 to 2021-04-07 08:32:11 +0000
- Make it easier to copy and paste example commands.  (rust-embedded/book#289)