Drive-by mention unsafe fn closure coercion

Author: Alexis Hunt

src/types/closure.md

@@ -131,8 +131,8 @@ more specific call traits:
 > not how it captures them.
 
 *Non-capturing closures* are closures that don't capture anything from their
-environment. They can be coerced to function pointers (`fn`) with the matching
-signature.
+environment. They can be coerced to function pointers (`fn` or `unsafe fn`)
+with the matching signature.
 
 ```rust
 let add = |x, y| x + y;