wined3d: Clarify WINED3DUSAGE_WRITEONLY a bit.

dlls/wined3d/resource.c

@@ -59,6 +59,12 @@ static void resource_check_usage(DWORD usage)
             | WINED3DUSAGE_OVERLAY
             | WINED3DUSAGE_TEXTURE;
 
+    /* WINED3DUSAGE_WRITEONLY is supposed to result in write-combined mappings
+     * being returned. OpenGL doesn't give us explicit control over that, but
+     * the hints and access flags we set for typical access patterns on
+     * dynamic resources should in theory have the same effect on the OpenGL
+     * driver. */
+
     if (usage & ~handled)
         FIXME("Unhandled usage flags %#x.\n", usage & ~handled);
     if ((usage & (WINED3DUSAGE_DYNAMIC | WINED3DUSAGE_WRITEONLY)) == WINED3DUSAGE_DYNAMIC)