docs: add details on updating a cart's customer (#10226)

Should be merged after releasing v2.0.5

Closes DX-1096

www/apps/resources/app/storefront-development/cart/update/page.mdx

@@ -46,3 +46,46 @@ fetch(`http://localhost:9000/store/carts/${cartId}`, {
 ```
 
 The Update Cart API route accepts a `region_id` request body parameter, whose value is the new region to associate with the cart.
+
+---
+
+## Set Cart's Customer
+
+You might need to change the cart's customer in two cases:
+
+- You created the cart for the customer as a guest, then they logged-in, so you want to associate the cart with them as a registered customer.
+- You're transfering the cart from one customer to another, which is useful in company setups, such as when implementing B2B commerce applicatons.
+
+To set or change the cart's customer, send an authenticated `POST` request to the Set Cart's Customer API route:
+
+<Note>
+
+This API route is only available after [Medusa v2.0.5](https://github.com/medusajs/medusa/releases/tag/v2.0.5).
+
+</Note>
+
+```ts
+fetch(`http://localhost:9000/store/carts/${cartId}/customer`, {
+  credentials: "include",
+  method: "POST",
+  headers: {
+    "Content-Type": "application/json",
+    
+  },
+})
+.then((res) => res.json())
+.then(({ cart }) => {
+  // use cart...
+  console.log(cart)
+})
+```
+
+To send an authenticated request, either use `credentials: include` if the customer is already authenticated with a cookie session, or pass the Authorization Bearer token in the request's header.
+
+<Note title="Tip">
+
+Learn more about authenticating customers in [this guide](../../customers/login/page.mdx).
+
+</Note>
+
+The cart is now associated with the logged-in customer.