Improve `std::thread::available_parallelism` docs
Improve `std::thread::available_parallelism` docs

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