Rollup merge of rust-lang#73675 - ehuss:update-books, r=ehuss

Update books

## reference

5 commits in 5d40ba5c2515caffa7790cda621239dc21ef5a72..04d5d5d7ba624b6f5016298451f3a63d557f3260
2020-06-06 20:25:36 -0700 to 2020-06-16 15:08:05 -0700
- Mention `feature="foo"` is a Cargo convention. (rust-lang/reference#833)
- fix out of date info on type aliases (rust-lang/reference#831)
- Fix an invalid variable name in the loop example (rust-lang/reference#832)
- Fix note about using proc_macro with Cargo. (rust-lang/reference#815)
- Add a link to the definition of Pattern_White_Space. (rust-lang/reference#824)

## book

7 commits in 30cd9dfe71c446de63826bb4472627af45acc9db..4e7c00bece1544d409312ec93467beb62b5bd0cb
2020-06-07 23:07:19 -0500 to 2020-06-19 09:39:12 -0400
- Link to the reference file that exists
- Link to the reference
- Clean up discussion around advanced lifetime stuff (rust-lang/book#2351)
- Reword Chapter 6 page 2, match (rust-lang/book#2374)
- Clarify some package/crate distinctions in chapter 14 (rust-lang/book#2373)
- Not mandatory with cargo 1.41.0-nightly (rust-lang/book#2368)
- Use same naming for Rhs as libcore/ops (rust-lang/book#2371)

## rust-by-example

4 commits in 7aa82129aa23e7e181efbeb8da03a2a897ef6afc..6f94ccb48da6fa4ed0031290f21411cf789f7d5e
2020-05-25 14:54:26 -0300 to 2020-06-20 17:51:30 -0300
- Update to mdbook 0.3.7 (rust-lang/rust-by-example#1352)
- Update fn.md (rust-lang/rust-by-example#1351)
- Fixed typo in formatted print (rust-lang/rust-by-example#1350)
- This explanation incorrectly inverts the meaning of SuperTrait (rust-lang/rust-by-example#1349)

## embedded-book

5 commits in 5555a97f04ad7974ac6fb8fb47c267c4274adf4a..616962ad0dd80f34d8b802da038d0aed9dd691bb
2020-05-25 18:00:51 +0000 to 2020-06-23 16:03:45 +0000
- Update RTFM name to RTIC, fixed links, updated singletons.md example.  (rust-embedded/book#254)
- Note on how to rebuild if memory.x is changed  (rust-embedded/book#253)
- Ease the readers into the Discovery book  (rust-embedded/book#250)
- Provide a note on 'extern crate' usage in edition 2018 syntax of Rust  (rust-embedded/book#248)
- Fix Typos and Improve Readability  (rust-embedded/book#245)