cgo: update documentation on calling C variadic functions

The current implementation does not support calling C variadic
functions (as discussed in #975). Document that.

Fixes #23537

Change-Id: If4c684a3d135f3c2782a720374dc4c07ea66dcbb
Reviewed-on: https://go-review.googlesource.com/90415Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cgo/doc.go

@@ -223,6 +223,26 @@ C compilers are aware of this calling convention and adjust
 the call accordingly, but Go cannot. In Go, you must pass
 the pointer to the first element explicitly: C.f(&C.x[0]).
 
+Calling variadic C functions is not supported. It is possible to
+circumvent this by using a C function wrapper. For example:
+
+	package main
+
+	// #include <stdio.h>
+	// #include <stdlib.h>
+	//
+	// static void myprint(char* s) {
+	//   printf("%s\n", s);
+	// }
+	import "C"
+	import "unsafe"
+
+	func main() {
+		cs := C.CString("Hello from stdio")
+		C.myprint(cs)
+		C.free(unsafe.Pointer(cs))
+	}
+
 A few special functions convert between Go and C types
 by making copies of the data. In pseudo-Go definitions: