docs: add a troubleshooting section on creating a payment session with zero total (#12974)

www/apps/api-reference/specs/store/paths/store_payment-collections_{id}_payment-sessions.yaml

@@ -2,11 +2,27 @@ post:
   operationId: PostPaymentCollectionsIdPaymentSessions
   summary: Initialize Payment Session of a Payment Collection
   x-sidebar-summary: Initialize Payment Session
-  description: >-
+  description: >
     Initialize and add a payment session to a payment collection. This is used
     during checkout, where you create a payment collection for the cart, then
     initialize a payment session for the payment provider that the customer
     chooses.
+
+    It's highly recommended to have an amount greater than `0` in the payment
+    collection, as some payment providers, such as Stripe, require a non-zero
+    amount to create a payment session. Otherwise, an error will be thrown on
+    the payment provider's side.
+
+    In cases where you want to create a payment session for a payment collection
+    with an amount of `0`, you can use the Manual System Payment Provider
+    instead of third-party payment providers. The Manual System Payment Provider
+    is built into Medusa and allows you to create payment sessions without
+    interacting with an external payment provider.
+
+    Make sure to configure the Manual System Payment Provider in your store's
+    region. Learn more in the [Manage
+    Region](https://docs.medusajs.com/user-guide/settings/regions#edit-region-details)
+    user guide.
   externalDocs:
     url: >-
       https://docs.medusajs.com/v2/resources/storefront-development/checkout/payment