it seems that this very useful function ChargeBee_Subscription::subscriptionsForCustomer is undocumented here https://apidocs.chargebee.com/docs/api
The List Subscriptions for a customer API (ChargeBee_Subscription::subscriptionsForCustomer) was a part of our v1 library. You can find the documentation in our v1 docs. Here's the link for it.
The API call has been deprecated in v2 since the List Subscriptions API could be used for the same purpose by passing the customer id filter param(customerId().<operator>(val)).
Do let us know if you have any further queries.