include note on feature unification

book/src/unsafe.md

@@ -23,6 +23,11 @@ undefined behavior. Additionally, code generated by the macro will not use the
 unsafe keyword, so generated code may be used in a crates using the 
 `#![forbid(unsafe_code)]` attribute.
 
+When the `forbid-unsafe` feature is added to a direct dependency on the `Logos` crate,
+[Feature Unification](https://doc.rust-lang.org/cargo/reference/features.html#feature-unification)
+ensures any transitive inclusion of `Logos` via other dependencies also have unsafe
+code disabled.
+
 Generally, disabling unsafe code will result in a slower parser.
 
 However making definitive statements around performance of safe-only code is difficult,