Rollup merge of rust-lang#78492 - ehuss:update-books, r=ehuss

Update books

## nomicon

7 commits in 6e57e64501f61873ab80cb78a07180a22751a5d6..69333eddb1de92fd17e272ce4677cc983d3bd71d
2020-09-14 11:40:23 -0400 to 2020-10-17 15:44:12 -0700
- Tweak GHA config (rust-lang/nomicon#240)
- Fix link for `[T]` (rust-lang/nomicon#239)
- Update casts.md (rust-lang/nomicon#232)
- [WIP] Add more links (rust-lang/nomicon#180)
- Data Race definition should be more precise (rust-lang/nomicon#219)
- Update the diagnostic of `error[E0597]` in dropck.md (rust-lang/nomicon#157)
- fix typo in Lifetimes mutable reference aliasing section (rust-lang/nomicon#225)

## reference

3 commits in 1b78182e71709169dc0f1c3acdc4541b6860e1c4..10c16caebe475d0d11bec0531b95d7697856c13c
2020-10-11 13:53:47 -0700 to 2020-10-25 20:51:26 -0700
- Add `unsafe` for `mod` and `extern`. (rust-lang/reference#898)
- mention how unions interact with dropping (rust-lang/reference#897)
- Add `move_ref_pattern` docs (rust-lang/reference#881)

## book

2 commits in 451a1e30f2dd137aa04e142414eafb8d05f87f84..13e1c05420bca86ecc79e4ba5b6d02de9bd53c62
2020-10-05 09:11:18 -0500 to 2020-10-20 14:57:32 -0500
- Referencing to Appendix B (rust-lang/book#2481)
- Use GITHUB_PATH instead of add-path (rust-lang/book#2477)

## rust-by-example

2 commits in 152475937a8d8a1f508d8eeb57db79139bc803d9..99eafee0cb14e6ec641bf02a69d7b30f6058349a
2020-10-09 09:29:50 -0300 to 2020-10-21 14:21:55 -0300
- Formatting footer items. (rust-lang/rust-by-example#1385)
- Add partial moves example for `move_ref_pattern` stabilization (rust-lang/rust-by-example#1377)

## edition-guide

3 commits in 81f16863014de60b53de401d71ff904d163ee030..7bc9b7a5e800f79df62947cb7d566fd2fbaf19fe
2020-08-27 13:56:31 -0700 to 2020-10-23 18:31:23 -0500
- A few small updates. (rust-lang/edition-guide#221)
- Clarify the limitation of ? in main and tests (rust-lang/edition-guide#219)
- Update deprecated GitHub Actions commands. (rust-lang/edition-guide#220)