[hud] Added some missing documentation

src/dxvk/hud/dxvk_hud.h

@@ -36,8 +36,21 @@ namespace dxvk::hud {
     
     ~Hud();
     
+    /**
+     * \brief Update HUD
+     * 
+     * Updates the data to display.
+     * Should be called once per frame.
+     */
     void update();
 
+    /**
+     * \brief Render HUD
+     * 
+     * Renders the HUD to the given context.
+     * \param [in] ctx Device context
+     * \param [in] surfaceSize Image size, in pixels
+     */
     void render(
       const Rc<DxvkContext>& ctx,
             VkExtent2D       surfaceSize);