comment for magic number

Co-authored-by: Ralf Jung <[email protected]>

tests/fail/shims/non_vararg_signature_mismatch.rs

@@ -10,6 +10,6 @@ extern "C" {
 
 fn main() {
     let c_path = CString::new(OsStr::new("./text").as_bytes()).expect("CString::new failed");
-    let _fd = unsafe { open(c_path.as_ptr(), 2) };
+    let _fd = unsafe { open(c_path.as_ptr(), /* value does not matter */ 0) };
     //~^ ERROR: calling a variadic function with a non-variadic caller-side signature
 }