Add comment on workspace-level lints

Author: taiki-e

Cargo.toml

@@ -24,3 +24,4 @@ missing_debug_implementations = "warn"
 rust_2018_idioms = "warn"
 single_use_lifetimes = "warn"
 unreachable_pub = "warn"
+# unsafe_op_in_unsafe_fn = "warn" # Set at crate-level instead since https://github.com/rust-lang/rust/pull/100081 is not available on MSRV