d3d9: Document a Microsoft header bug.

include/d3d9.h

@@ -241,7 +241,15 @@ DECLARE_INTERFACE_(IDirect3D9Ex,IDirect3D9)
     STDMETHOD_(ULONG,AddRef)(THIS) PURE;
     STDMETHOD_(ULONG,Release)(THIS) PURE;
     /*** IDirect3D9 methods ***/
+
+    /* Note: Microsoft's d3d9.h does not declare IDirect3D9Ex::RegisterSoftwareDevice . This would mean that
+     * the offsets of the other methods in the Vtable change too. This is wrong. In Microsoft's
+     * d3d9.dll, the offsets for the other functions are still compatible with IDirect3D9.
+     * This is probably because even in MS's header IDirect3D9Ex inherits from IDirect3D9, which makes the
+     * C++ inferface compatible, and nobody uses the C interface in Windows world.
+     */
     STDMETHOD(RegisterSoftwareDevice)(THIS_ void* pInitializeFunction) PURE;
+
     STDMETHOD_(UINT, GetAdapterCount)(THIS) PURE;
     STDMETHOD(GetAdapterIdentifier)(THIS_ UINT Adapter, DWORD Flags, D3DADAPTER_IDENTIFIER9* pIdentifier) PURE;
     STDMETHOD_(UINT, GetAdapterModeCount)(THIS_ UINT Adapter, D3DFORMAT Format) PURE;