Rollup merge of #80973 - ehuss:update-books, r=ehuss

Update books

## nomicon

2 commits in a5a48441d411f61556b57d762b03d6874afe575d..a8584998eacdea7106a1dfafcbf6c1c06fcdf925
2020-12-06 10:39:41 +0900 to 2021-01-06 12:49:49 -0500
- Update vector code examples
- Remove outdated information about `jemalloc`

## reference

13 commits in b278478b766178491a8b6f67afa4bcd6b64d977a..50af691f838937c300b47812d0507c6d88c14f97
2020-12-21 18:18:03 -0800 to 2021-01-12 21:19:20 -0800
- Update grammar for parser unification. (rust-lang/reference#927)
- Define constraining an implementation (rust-lang/reference#928)
- Document extra behavior of #[no_mangle] (rust-lang/reference#930)
- Add a float examle without a `.`. (rust-lang/reference#929)
- Add more details about const generics. (rust-lang/reference#921)
- Fix footnotes. (rust-lang/reference#926)
- Add "Logic errors" as behavior not considered unsafe (rust-lang/reference#919)
- Update grammar for order of parameters/arguments. (rust-lang/reference#920)
- Fix formatting in the tuple section (rust-lang/reference#923)
- document const generics (rust-lang/reference#901)
- Update mdbook (rust-lang/reference#918)
- linkage.md: update link to FFI section of the Book. (rust-lang/reference#917)
- Document array expression with a const. (rust-lang/reference#914)

## book

8 commits in 5bb44f8b5b0aa105c8b22602e9b18800484afa21..ac57a0ddd23d173b26731ccf939f3ba729753275
2020-12-18 20:07:31 -0500 to 2021-01-09 14:18:45 -0500
- Update version of mdbook we're testing with to 0.4.5 (rust-lang/book#2561)
- Fix grammar in ch13-01-closures.md (rust-lang/book#2534)
- Merge remote-tracking branch 'origin/pr/2527'
- Clarify code example ch6.3 (rust-lang/book#2485)
- Fix link added in rust-lang/book#2495 to be relative and at the bottom
- Merge remote-tracking branch 'origin/pr/2495'
- Update output to match the updated poem punctuation
- Fix rust-lang/book#2539 - Remove fancy apostrophes from poem for Windows

## rust-by-example

3 commits in 1cce0737d6a7d3ceafb139b4a206861fb1dcb2ab..03e23af01f0b4f83a3a513da280e1ca92587f2ec
2020-12-21 17:36:29 -0300 to 2021-01-09 10:20:28 -0300
- Replace for loop with iteration (rust-lang/rust-by-example#1404)
- Update mdbook (rust-lang/rust-by-example#1402)
- Add note for match guards to include catch-all (rust-lang/rust-by-example#1401)

## embedded-book

1 commits in ba34b8a968f9531d38c4dc4411d5568b7c076bfe..ceec19e873be87c6ee5666b030c6bb612f889a96
2020-11-17 00:20:43 +0000 to 2021-01-03 13:13:10 +0000
- book.toml: add link to GitHub repo  (rust-embedded/book#276)

src/doc/book

@@ -1 +1 @@
-Subproject commit 5bb44f8b5b0aa105c8b22602e9b18800484afa21
+Subproject commit ac57a0ddd23d173b26731ccf939f3ba729753275

src/doc/embedded-book

@@ -1 +1 @@
-Subproject commit ba34b8a968f9531d38c4dc4411d5568b7c076bfe
+Subproject commit ceec19e873be87c6ee5666b030c6bb612f889a96

src/doc/nomicon

@@ -1 +1 @@
-Subproject commit a5a48441d411f61556b57d762b03d6874afe575d
+Subproject commit a8584998eacdea7106a1dfafcbf6c1c06fcdf925

src/doc/reference

@@ -1 +1 @@
-Subproject commit b278478b766178491a8b6f67afa4bcd6b64d977a
+Subproject commit 50af691f838937c300b47812d0507c6d88c14f97

src/doc/rust-by-example

@@ -1 +1 @@
-Subproject commit 1cce0737d6a7d3ceafb139b4a206861fb1dcb2ab
+Subproject commit 03e23af01f0b4f83a3a513da280e1ca92587f2ec