Rollup merge of #161321 - rustbot:docs-update, r=traviscross

Update books

## rust-lang/edition-guide

2 commits in 5a14f7276ebc0bd100974f02b918f30472c782fc..f5abcf137698e5ad6ebed359d69654ff705346af
2026-08-18 19:42:43 UTC to 2026-08-11 17:44:56 UTC

- ignore tests which will be broken by never type stabilization (rust-lang/edition-guide#383)
- Allow override inherited default-features (rust-lang/edition-guide#387)

## rust-lang/nomicon

1 commits in 5012a37c682b26c4e19433888ed2ca9b129696ca..5791ca9f5d671328af7a8fe87b42ca90c7211d28
2026-08-17 09:48:34 UTC to 2026-08-17 09:48:34 UTC

- Remove obsolete improperly reduced borrows example (rust-lang/nomicon#526)

## rust-lang/reference

6 commits in 82da570bafd9efed421bdbae0f8603ede8dd308b..eda708334abad285d63bc1f3558a51e8b790346e
2026-08-18 19:47:06 UTC to 2026-08-11 17:18:06 UTC

- Revert "Document RISC-V d, e, and f extensions" (rust-lang/reference#2329)
- Bump railroad-dependency; themed syntax diagrams (rust-lang/reference#2328)
- document naked c-variadic functions (rust-lang/reference#2321)
- Add `extern "custom"` (rust-lang/reference#2300)
- Empty repr(Rust) enums are ZSTs (rust-lang/reference#2293)
- aliasing rule summary: use a bulleted list (rust-lang/reference#2322)

Author: jhpratt