Clarify `post_install_msg`s advice
Clarify `post_install_msg`s advice

Someone who supplies a `no_modify_path` option requires less clarification.
rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs