Mention Variadics With No Fixed Parameter

There's an open PR to support C23's variadics without a named
parameter in Rust's extern blocks. Currently, it's waiting on
approval from T-lang to get merged.

This PR removes the line "There must be at least one parameter
before the variadic parameter" and adds an example to highlight
that we support variadics without any named parameters.

Opening this now so it can get merged after the implementation
gets approved by T-lang.

Author: veera-sivarajan

src/items/external-blocks.md

@@ -97,13 +97,13 @@ There are also some platform-specific ABI strings:
 ## Variadic functions
 
 Functions within external blocks may be variadic by specifying `...` as the
-last argument. There must be at least one parameter before the variadic
-parameter. The variadic parameter may optionally be specified with an
+last argument. The variadic parameter may optionally be specified with an
 identifier.
 
 ```rust
 extern "C" {
-    fn foo(x: i32, ...);
+    fn foo(...);
+    fn bar(x: i32, ...);
     fn with_name(format: *const u8, args: ...);
 }
 ```