Rollup merge of #28463 - critiqjo:book-concurrency, r=steveklabnik
Rollup merge of #28463 - critiqjo:book-concurrency, r=steveklabnik

Fixes #28458

Details about `Arc` may be too soon to be described together with `Sync`... Tell me what you think.

r? @steveklabnik 

aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust