Add a concrete example of CStruct initialization

doc/Language/nativecall.pod6

@@ -286,7 +286,22 @@ type of a native function, the memory is not managed for you by the GC.
 
 NativeCall currently doesn't put object members in containers, so assigning new values
 to them (with =) doesn't work. Instead, you have to bind new values to the private
-members: C<$!struct-member := StructObj.new;>
+members:
+
+    class MyStruct is repr('CStruct') {
+        has CArray[num64] $!arr;
+        has Str $!str;
+        has IntStruct $!int-struct;
+
+        submethod TWEAK {
+            my $arr := CArray[num64].new;
+            $arr[0] = 0.9e0;
+            $arr[1] = 0.2e0;
+            $!arr := $arr;
+            $!str := 'Perl 6 is fun';
+            $!int-struct := IntStruct.new;
+        }
+    }
 
 As you may have predicted by now, a null is represented by the type object of the
 struct type.