Updated Implementation (markdown)

Implementation.md

@@ -34,6 +34,11 @@ The ``std::function`` is used for callbacks as a general pattern so that client
     #endif
     #endif
 
+When declaring these parameters, we use the macro:
+
+    HRESULT __cdecl SaveWICTextureToFile( /*...*/,
+        std::function<void DIRECTX_STD_CALLCONV(IPropertyBag2*)> setCustomProps = nullptr );
+
 _Note: ``std::function`` doesn't support using ``__vectorcall`` until VS 2015, so use of ``/Gv`` is difficult in VS 2013 or earlier._
 
 # Default constructors/assignment operators