Update stock items article after PR review

This clarifies some information about `Spree::StockItem`s after a
review by kennyadsl. Specifically, it confirms how many `StockItem`s
should exist if there are multiple `StockLocation`s, and it details how
backorderable stock items are handled.

guides/inventory/stock-items.md

@@ -1,8 +1,15 @@
 # Stock items
 
-On-hand inventory is tracked using the `Spree::StockItem` model. Each
-`Spree::Variant` in a store has a corresponding `Spree::StockItem` object with a
-`count_on_hand` value that represents the number of items you have in stock.
+On-hand inventory is tracked using the `Spree::StockItem` model. A stock item
+tracks stock at a single `Spree::StockLocation`.
+
+If you only track stock at one stock location, then every `Spree::Variant` in
+your store has one corresponding `Spree::StockItem` object. If you track stock
+at two stock locations, then every `Spree::Variant` in your store has *two*
+corresponding `Spree::StockItem`s: one for each `Spree::StockLocation`.
+
+The `Spree::StockItem`'s `count_on_hand` value that represents the number of
+items you have in stock.
 
 `Spree::StockItem` objects have the following attributes:
 
@@ -32,10 +39,9 @@ While a `Spree::StockMovement` object logs the increase or decrease of the
 `solidus_backend`.
 
 Whenever an administrator updates the count on hand, they are discarding the old
-value completely. So, if a stock item is
-[backorderable](#backorderable-stock-items) and its `count_on_hand` is `-5`,
-when the administrator changes the value to `20`, they are creating a
-`Spree::StockMovement` with a value of `25`.
+value completely. So, if a stock item's `count_on_hand` is `5`, when the
+administrator changes the value to `20`, they are creating a
+`Spree::StockMovement` with a value of `15`.
 
 See the [Stock movements][stock-movements] article for more information.
 
@@ -45,4 +51,16 @@ See the [Stock movements][stock-movements] article for more information.
 
 If a `Spree::StockItem` is `backorderable`, then customers can continue to order
 it after the product is sold out. When a sold out product continues to sell, the
-`count_on_hand` would become a negative integer.
+`count_on_hand` becomes a negative integer.
+
+For example, if a customer orders five backorderable items and its
+`count_on_hand` becomes `-5`, the customer can still check out successfully.
+[Inventory units][inventory-units] with the `state` value of `backordered` are
+created for the five items.
+
+The `Spree::Shipment`(s) associated with the backordered items cannot be shipped
+until the stock has been replenished. Once the item is in stock again, each
+backordered inventory unit's `state` value is changed from `backordered` to
+`on_hand` and the shipment becomes shippable.
+
+[inventory-units]: inventory-units.md