Make discussion of flow identifiers more vague

[DavidSchinazi]

From @martinthomson (link to email):

The bit that mentions flow identifiers shouldn't presume so much about how these frames will be used. The only point you need to make is that it is up to the protocol in use to define how the receiver of a DATAGRAM frame processes that frame. If you are going to make a point, you might observe that it might be desirable in some protocols to have multiple logical entities receiving DATAGRAM frames, in which case it is up to the protocol to define how DATAGRAM frames are efficiently routed to one of those entities.