Document opt-level 's' and 'z' stabilized in 1.28.
Document opt-level 's' and 'z' stabilized in 1.28.

nipunn1313/cargo,tsurai/cargo,rust-lang/cargo,eminence/cargo,tsurai/cargo,rust-lang/cargo,vojtechkral/cargo,nipunn1313/cargo,eminence/cargo,rust-lang/cargo,tsurai/cargo,rustyhorde/cargo,rustyhorde/cargo,tsurai/cargo,vojtechkral/cargo,tshepang/cargo,rustyhorde/cargo,vojtechkral/cargo,nipunn1313/cargo,eminence/cargo,rustyhorde/cargo,rust-lang/cargo,nipunn1313/cargo,tshepang/cargo,tshepang/cargo,tshepang/cargo,vojtechkral/cargo,eminence/cargo,tsurai/cargo