clarify that rust_pkg is a place-holder

the switch from package `hello` to `rust_pkg` is a little jarring; I'd use <var> but I don't see how. ALL_CAPS seems like a reasonable  poor-man's <var>.
auto merge of #9645 : dckc/rust/patch-2, r=catamorphism

the switch from package `hello` to `pkg_id` is a little jarring; I'd use `<var>` but I don't see how. ALL_CAPS i.e. PKG_ID seems like a reasonable  poor-man's `<var>`.