Update prefix and documentation.
diff --git a/include/D3D12MemAlloc.h b/include/D3D12MemAlloc.h
index 6b11a99..4bf64ee 100644
--- a/include/D3D12MemAlloc.h
+++ b/include/D3D12MemAlloc.h
@@ -59,9 +59,12 @@
 */

 

 // If using this library on a platform different than Windows PC or want to use different version of DXGI,

-// you should include D3D12-compatible headers before this library on your own and define this macro.

+// you should include D3D12-compatible headers before this library on your own and define 

+// D3D12MA_D3D12_HEADERS_ALREADY_INCLUDED.

+// Alternatively, if you are targeting the open sourced DirectX headers, defining D3D12MA_USING_DIRECTX_HEADERS

+// will include them rather the ones provided by the Windows SDK.

 #ifndef D3D12MA_D3D12_HEADERS_ALREADY_INCLUDED

-    #if defined(USING_DIRECTX_HEADERS)

+    #if defined(D3D12MA_USING_DIRECTX_HEADERS)

         #include <directx/d3d12.h>

         #include <dxguids/dxguids.h>

     #else