Add working example for E0617 explanation

src/librustc_error_codes/error_codes/E0617.md

@@ -17,3 +17,14 @@ Certain Rust types must be cast before passing them to a variadic function,
 because of arcane ABI rules dictated by the C standard. To fix the error,
 cast the value to the type specified by the error message (which you may need
 to import from `std::os::raw`).
+
+In this case, `c_double` has the same size as `f64` so we can use it directly:
+
+```
+# extern {
+#     fn printf(c: *const i8, ...);
+# }
+unsafe {
+    printf(::std::ptr::null(), 0f64); // ok!
+}
+```