Update documentation with respect to implementation status IN_ADVANCE mode

[akumar-xyz]

At userguide/subscription/includes/catalog-usage.adoc, the language implies the IN_ADVANCE mode is yet to bee implemented

We plan to add the IN_ADVANCE mode in the future to support prepaid scenarios such as toll bridge pass

However there are examples within the docs which explain how to use the IN_ADVANCE mode.

The the catalog-usage document may require updating to prevent confusion.

I have only gone through the docs and haven't used killbill extensively yet. So it is definitely possible that I might have missed something here