docs: add missing details for tiered pricing (#12374)

* docs: add missing details for tiered pricing

* adjustments

www/apps/resources/app/commerce-modules/pricing/price-calculation/page.mdx

@@ -193,6 +193,11 @@ const priceSet = await pricingModuleService.createPriceSets({
         region_id: "reg_123",
       },
     },
+    {
+      amount: 200,
+      currency_code: "EUR",
+      min_quantity: 100,
+    }
   ],
 })
 ```
@@ -329,6 +334,83 @@ const priceSet = await pricingModuleService.createPriceSets({
   </TabsContentWrapper>
 </Tabs>
 
+### Tiered Pricing Selection
+
+<Tabs defaultValue="code">
+  <TabsList>
+    <TabsTrigger value="code">Code</TabsTrigger>
+    <TabsTrigger value="result">Result</TabsTrigger>
+  </TabsList>
+  <TabsContentWrapper>
+    <TabsContent value="code">
+
+    ```ts
+    const price = await pricingModuleService.calculatePrices(
+      { id: [priceSet.id] },
+      {
+        context: {
+          cart: {
+            items: [
+              {
+                id: "item_1",
+                quantity: 200,
+                // assuming the price set belongs to this variant
+                variant_id: "variant_1",
+                // ...
+              }
+            ],
+            // ...
+          }
+        }
+      }
+    )
+    ```
+
+    </TabsContent>
+    <TabsContent value="result">
+
+    The returned price is:
+
+    ```ts
+    const price = {
+      id: "<PRICE_SET_ID>",
+      is_calculated_price_price_list: false,
+      calculated_amount: 200,
+
+      is_original_price_price_list: false,
+      original_amount: 200,
+
+      currency_code: "EUR",
+
+      is_calculated_price_tax_inclusive: false,
+      is_original_price_tax_inclusive: false,
+
+      calculated_price: {
+        price_id: "<DEFAULT_PRICE_ID>",
+        price_list_id: null,
+        price_list_type: null,
+        min_quantity: 100,
+        max_quantity: null,
+      },
+
+      original_price: {
+        price_id: "<DEFAULT_PRICE_ID>",
+        price_list_id: null,
+        price_list_type: null,
+        min_quantity: 100,
+        max_quantity: null,
+      },
+    }
+    ```
+
+    - Original price selection: the fifth price in the price list is selected as the best price because the cart item quantity is 200.
+        - This is assuming the price set belongs to the cart item's variant.
+    - Calculated price selection: since there are no associated price lists, the calculated price is set to the original price.
+
+    </TabsContent>
+  </TabsContentWrapper>
+</Tabs>
+
 ### Price Selection with Price List
 
 <Tabs defaultValue="code">