Update billing-settings-embed.md

Author: bsears90

docs/billing-settings-embed.md

@@ -20,6 +20,7 @@ In the init for a request embed, these are the options which can customize the e
 | handleResponse | function(response) | a function which gets called after billing operations occur |
 | token | string | A JSON web token for the user account being managed (generated by the server code) |
 | [disablePlanChange] | boolean | if true, a customer will not be able to change their plan using the embeddable |
+| [serviceInstanceId] | string | If specified, only pull up the billing settings for this specific instance, useful if your customers can have multiple subscriptions and you don't want them to see all of them on one page |
 
 ## Example Billing Settings Config
 ```javascript