Need to Document BridgeableStore

[DennisMcWherter]

The bridgeable store remains undocumented today. This is unfortunate because it enables Elide to coordinate across datastores with compound models (i.e. a hibernate persisted-model can have a relationship in Elide to a redis-persisted model, for instance).

We need to document the what, why, and how. Fortunately, the interface itself is well-documented: https://github.com/yahoo/elide/blob/master/elide-datastore/elide-datastore-multiplex/src/main/java/com/yahoo/elide/datastores/multiplex/BridgeableTransaction.java