specify that extern c-variadic functions cannot be safe

Author: folkertdev

src/items/external-blocks.md

@@ -182,6 +182,11 @@ unsafe extern "C" {
 }
 ```
 
+r[items.extern.variadic.safety]
+
+A variadic extern function cannot be qualified as `safe`.
+Passing an unexpected number of arguments or arguments of an unexpected type to a variadic function is UB.
+
 r[items.extern.attributes]
 ## Attributes on extern blocks