Rollup merge of #38013 - wezm:simplify-test-notes, r=steveklabnik
Rollup merge of #38013 - wezm:simplify-test-notes, r=steveklabnik

Simplify notes on testing and concurrency

The start of the notes on tests running concurrently, added in https://github.com/rust-lang/rust/pull/37766 read a little awkwardly. This PR fixes that and simplifies the wording a bit.

r? @steveklabnik

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