DEVDOCS-6662 - Add product_description model to app extention's (#1192)

<!-- Ticket number or summary of work -->
# [DEVDOCS-6662]


## What changed?
<!-- Provide a bulleted list in the present tense -->
* The App Extensions Glossary was missing a 4th model
`PRODUCT_DESCRIPTION`. This Add's in the details for the
`PRODUCT_DESCRIPTION` model.

## Release notes draft
* Added missing 4th model `PRODUCT_DESCRIPTION` in the [App
Extensions](https://developer.bigcommerce.com/docs/integrations/apps/app-extensions/glossary#model)
glossary.

ping @bc-terra 


[DEVDOCS-6662]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6662?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: chris-nowicki

docs/integrations/apps/app-extensions/glossary.mdx

@@ -18,6 +18,7 @@ An App Extension's **model** describes the control panel view it extends. Curren
 | Enum | Description |
 |:-----|:------------|
 | `PRODUCTS` | App Extension menu items render in the **Action** menu for each product on the **[View Products](https://login.bigcommerce.com/deep-links/manage/products)** page. |
+| `PRODUCT_DESCRIPTION` | App Extension menu items render in the **Action** menu within Description section on the **[Edit a Product](https://login.bigcommerce.com/deep-links/manage/products/edit)** page. |
 | `ORDERS` | App Extension menu items render in the **Action** menu for each order on the **[View Orders](https://login.bigcommerce.com/deep-links/manage/orders)** page. |
 | `CUSTOMERS` | App Extension menu items render in the **Action** menu for each customer on the **[View Customers](https://login.bigcommerce.com/deep-links/manage/customers)** page. |