update for more ABIs supporting c-variadics

Author: RalfJung

src/items/external-blocks.md

@@ -187,6 +187,18 @@ unsafe extern "C" {
 > [!WARNING]
 > The `safe` qualifier should not be used on a function in an `extern` block unless that function guarantees that it will not access the variadic arguments at all. Passing an unexpected number of arguments or arguments of unexpected type to a variadic function may lead to [undefined behavior][undefined].
 
+r[items.extern.variadic.conventions]
+Variadic parameters can only be specified with [`extern`] blocks with the following ABI strings:
+- `"aapcs"`
+- `"C"`
+- `"cdecl"`
+- `"efiapi"`
+- `"system"`
+- `"sysv64"`
+- `"win64"`
+
+This also includes the corresponding [`-unwind` variants][items.fn.extern.unwind].
+
 r[items.extern.attributes]
 ## Attributes on extern blocks
 

src/types/function-pointer.md

@@ -51,9 +51,7 @@ function], and the `extern` qualifier indicates it is an [extern function].
 
 r[type.fn-pointer.constraint-variadic]
 Variadic parameters can only be specified with [`extern`] function types with
-the `"C"` or `"cdecl"` calling convention.
-
-This also includes the corresponding [`-unwind` variants][items.fn.extern.unwind].
+supported calling conventions, as defined [here][items.extern.variadic.conventions].
 
 r[type.fn-pointer.attributes]
 ## Attributes on function pointer parameters