Tweak orders documentation content

guides/orders/order-state-machine.md

@@ -16,7 +16,7 @@ There are six order states by default:
 If you go through the checkout provided in the `solidus_frontend` gem, you can
 see that there is a clearly defined step for each of these states during checkout.
 
-The `payment` state is optional and it's only triggered if `payment_required?` 
+The `payment` state is optional and it's only triggered if `payment_required?`
 returns `true`.
 
 The `complete` state is triggered in one of two ways:
@@ -47,7 +47,7 @@ def transition_forward
  @order.next
  end
 end
-``` 
+```
 
 If `next` returns `false`, then the order does not meet the criteria and does
 not transition to the next state.
@@ -57,13 +57,13 @@ not transition to the next state.
 ## Payments and shipments have their own state machines
 
 Note that a `Spree::Order` with the state `complete` does not mean that the
-payment has been processed or the order shipments have been shipped. See also the
-values of the `payment_state` and `shipment_state` attributes.
+payment has been processed or the order shipments have been shipped. See also
+the values of the [`payment_state`][payment-states] and
+[`shipment_state`][shipment-states] attributes.
 
 <!-- TODO:
- Once it's merged, link to documentation about checkout. This section only
- attempts to summarize some of what happens during the checkout flow.
-
- It may be useful to link to the payments documentation once it's merged, too.
+ Documentation about the checkout flow could be extended.
 -->
 
+[payment-states]: ../payments/overview.md#payment-states
+[shipment-states]: ../shipments/overview.md#shipment-states