[ADD] point_of_sale: How to register multiple orders simultaneously?

https://docs.google.com/document/d/1zWP_g17dv_JWOjS7KGje9Jl8NdR6knWYd7NuQ3aLJyc/edit?usp=sharing

point_of_sale/restaurant.rst

@@ -11,3 +11,4 @@ Advanced Restaurant Features
    restaurant/print
    restaurant/tips
    restaurant/transfer
+   restaurant/multi_orders

point_of_sale/restaurant/multi_orders.rst

@@ -0,0 +1,36 @@
+===============================================
+How to register multiple orders simultaneously?
+===============================================
+
+Register simultaneous orders
+============================
+
+On the main screen, just tap on the **+** on the top of the screen to
+register another order. The current orders remain opened until the
+payment is done or the order is cancelled.
+
+.. image:: media/multi_orders01.png
+    :align: center
+
+Switch from one order to another
+================================
+
+Simply click on the number of the order.
+
+.. image:: media/multi_orders02.png
+    :align: center
+
+Cancel an order
+===============
+
+If you made a mistake or if the order is cancelled, just click on the **-**
+button. A message will appear to confirm the order deletion.
+
+.. image:: media/multi_orders03.png
+    :align: center
+
+
+.. seealso::
+    * :doc:`../advanced/register`
+    * :doc:`../advanced/reprint`
+    * :doc:`transfer`