Add turbofish comment

Author: Sa4dUs

compiler/rustc_builtin_macros/src/autodiff.rs

@@ -536,6 +536,7 @@ mod llvm_enzyme {
 
     // Generate turbofish expression from fn name and generics
     // Given `foo` and `<A, B, C>` params, gen `foo::<A, B, C>`
+    // We use this expression when passing primal and diff function to the autodiff intrinsic
     fn gen_turbofish_expr(
         ecx: &ExtCtxt<'_>,
         ident: Ident,