Rollup merge of rust-lang#77951 - ehuss:update-books, r=ehuss

Update books

## reference

5 commits in 56a13c082ee90736c08d6abdcd90462517b703d3..1b78182e71709169dc0f1c3acdc4541b6860e1c4
2020-09-14 23:20:16 -0700 to 2020-10-11 13:53:47 -0700
- Specify that SSE4.1 includes SSSE3 instead of SSE3 (rust-lang/reference#892)
- Fix mutable expressions that can be dereferenced (rust-lang/reference#890)
- Fix grammar in memory model (rust-lang/reference#889)
- Add style checks. (rust-lang/reference#886)
- Add description for LUB Coercion (rust-lang/reference#808)

## book

1 commits in cb28dee95e5e50b793e6ba9291c5d1568d3ad72e..451a1e30f2dd137aa04e142414eafb8d05f87f84
2020-09-09 10:06:00 -0500 to 2020-10-05 09:11:18 -0500
- clarify description of when ? can be used (rust-lang/book#2471)

## rust-by-example

1 commits in 7d3ff1c12db08a847a57a054be4a7951ce532d2d..152475937a8d8a1f508d8eeb57db79139bc803d9
2020-09-28 15:54:25 -0300 to 2020-10-09 09:29:50 -0300
- Add 1.45.0 cast documentation (rust-lang/rust-by-example#1384)

## embedded-book

2 commits in dd310616308e01f6cf227f46347b744aa56b77d9..79ab7776929c66db83203397958fa7037d5d9a30
2020-09-26 08:54:08 +0000 to 2020-10-12 08:00:05 +0000
- llvm-objdump: Use two hyphens in flags to objdump  (rust-embedded/book#270)
- Start/hardware: clarify which file needs tweaking  (rust-embedded/book#266)

Author: JohnTitor