STRF-12557: Update Customer Login SSO documentation for new "enable_p…

…ersistent_cart" field (#549)

# [STRF-12557]

### Don't merge until this PR is fully released:
bigcommerce/bigcommerce#60116

## What changed?
* Updated Customer SSO to allow passing a `enable_persistent_cart` field
which determines whether or not to merge carts during customer SSO login
(when Persistent Cart is enabled)

## Release notes draft
* Added Customer SSO support for Persistent Carts

## Anything else?
n/a

ping @bigcommerce/team-storefront 


[STRF-12557]:
https://bigcommercecloud.atlassian.net/browse/STRF-12557?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

docs/start/authentication/customer-login.mdx

@@ -42,6 +42,7 @@ JWT is an industry standard ([RFC 7519](https://tools.ietf.org/html/rfc7519)) fo
 | `customer_id` | integer | The ID of the shopper who is signing in.|
 | `redirect_to` | string | Optional field containing a relative path for the shopper's destination after sign-in. Defaults to `/account.php`. |
 | `request_ip` | string | Optional field containing the expected IP address for the request. If provided, BigCommerce will check that it matches the browser trying to sign in.|
+| `enable_persistent_cart` | boolean | Optional field to specify whether or not to use persistent cart functionality on customer token login. This feature depends on you enabling the [Persistent Cart](https://support.bigcommerce.com/s/article/Persistent-Cart) setting on your store. The default value is `false`.|
 
 ## Prerequisites