feat(billing): BRP-17797 add additional field descriptions for UB (#1093)

<!-- Ticket number or summary of work -->
# [BRP-17797]
https://bigcommercecloud.atlassian.net/browse/BRP-17797


## What changed?
<!-- Provide a bulleted list in the present tense -->
* Added field descriptions for Unified Billing examples

## Release notes draft
<!-- Provide an entry for the release notes using simple, conversational
language. Don't be too technical. Explain how the change will benefit
the merchant and link to the feature.

Examples:
* The newly-released [X feature] is now available to use. Now, you’ll be
able to [perform Y action].
* We're happy to announce [X feature], which can help you [perform Y
action].
* [X feature] helps you to create [Y response] using the [Z query
parameter]. Now, you can deliver [ex, localized shopping experiences for
your customers].
* Fixed a bug in the [X endpoint]. Now the [Y field] will appear when
you click [Z option]. -->
* Added field descriptions for Unified Billing examples

## Anything else?
<!-- Add related PRs, salient notes, additional ticket numbers, etc. -->

ping {names}


[BRP-17797]:
https://bigcommercecloud.atlassian.net/browse/BRP-17797?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: Tom Ho

docs/integrations/apps/unified-billing/example-queries.mdx

@@ -64,6 +64,22 @@ The merchant will not be charged during this period. A subscription only generat
 
 Checkout links are valid for 24 hours and expire automatically to prevent misuse.
 
+#### Field Descriptions
+| Field | Description
+|:--------|:-----------|
+| accountId | The account UUID of the merchant you want to create a checkout for. |
+| id (product) | The id of your app. |
+| type (product) | The type of product. In the case for creating an app checkout, this will be `"APPLICATION"` |
+| productLevel (product) | The level of your product. This will help differentiate which plan the subscription is for in case your app supports different plan levels (like "Standard", "Plus", "Pro", "Silver", "Gold", "Premium", etc). |
+| id (scope) | Since the type of scope used here will be `STORE`, the scope id will be the store hash of the merchant you are sending the checkout to. |
+| type (scope) | The type of scope. In this case for creating an app checkout, this will be `"STORE"`. |
+| interval (pricingPlan) | The interval at which the merchant should be billed. This can be `"ANNUAL"`, `"MONTH"`, `"ONCE"`, `"QUARTER"`, or `"SEMIANNUAL"`. |
+| value (price) | The amount the merchant will be charged at each interval |
+| currencyCode (price) | The currency in which the merchant will be charged. Currently, only `USD` is supported. |
+| trialDays (pricingPlan) | The number of trial days granted to the merchant. The merchant will not be charged until the trial days are over. |
+| description | A brief description for your app |
+| redirectUrl | The URL the merchant will be redirected to after completing the checkout. |
+
 To create a checkout, run the `createCheckout` mutation:
 
 <Tabs items={["Request", "Sample variables", "Response"]}>
@@ -363,6 +379,18 @@ Once the checkout is created, you can periodically poll the status of the checko
 
 The `createCheckout` mutation can also be used to update an existing subscription's pricing plan information and product level. A `subscriptionId` must be passed in the request for this to be processed as an update.
 
+#### Field Descriptions
+| Field | Description
+|:--------|:-----------|
+| accountId | The account UUID of the merchant you want to create a checkout for. |
+| productLevel (product) | The level of your product. This will help differentiate which plan the subscription is for in case your app supports different plan levels (like "Standard", "Plus", "Pro", "Silver", "Gold", "Premium", etc). |
+| interval (pricingPlan) | The interval at which the merchant should be billed. This can be `"ANNUAL"`, `"MONTH"`, `"ONCE"`, `"QUARTER"`, or `"SEMIANNUAL"`. | |
+| value (price) | The amount the merchant will be charged at each interval |
+| currencyCode (price) | The currency in which the merchant will be charged. Currently, only `USD` is supported. |
+| description | A brief description for your app |
+| redirectUrl | The URL the merchant will be redirected to after completing the checkout. |
+| subscriptionId | The id of the subscription that was returned after a fetch of a completed checkout |
+
 <Tabs items={["Request", "Sample variables", "Response"]}>
   <Tab>