Copy core/alloc check-cfg message also in std

Author: Urgau

library/std/Cargo.toml

@@ -100,6 +100,9 @@ test = true
 
 [lints.rust.unexpected_cfgs]
 level = "warn"
+# x.py uses beta cargo, so `check-cfg` entries do not yet take effect
+# for rust-lang/rust. But for users of `-Zbuild-std` it does.
+# The unused warning is waiting for rust-lang/cargo#13925 to reach beta.
 check-cfg = [
     'cfg(bootstrap)',
     'cfg(target_arch, values("xtensa"))',