Update warning about linker-plugin-lto not working with -Os/-Oz
This was fixed in rust 1.49 already, fortunately.  But as the MSRV is
still 1.38 we cannot drop the warning entirely yet.
//! - You have to use a Rust version whose LLVM version is compatible with the toolchain in
//! `asm-toolchain`.
//!
-//! - Due to a [Rust bug][rust-lang/rust#75940], this option does not work with optimization
-//! levels `s` and `z`.
+//! - Due to a [Rust bug][rust-lang/rust#75940] in compiler versions **before 1.49**, this option
+//! does not work with optimization levels `s` and `z`.
//!
//! [Linker-Plugin LTO]: https://doc.rust-lang.org/stable/rustc/linker-plugin-lto.html
//! [rust-lang/rust#75940]: https://github.com/rust-lang/rust/issues/75940
