Fix cls_double_va.c and update docs

testsuite/libffi.call/cls_double_va.c

@@ -50,6 +50,9 @@ int main (void)
 	printf("res: %d\n", (int) res);
 	// { dg-output "\nres: 4" }
 
+	/* The call to cls_double_va_fn is static, so have to use a normal prep_cif */
+	CHECK(ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 2, &ffi_type_sint, arg_types) == FFI_OK);
+
 	CHECK(ffi_prep_closure_loc(pcl, &cif, cls_double_va_fn, NULL, code) == FFI_OK);
 
 	res	= ((int(*)(char*, double))(code))(format, doubleArg);