Martin's issue 7- Describe classification/matching

[rgwilton]

o ietf-if-l3-vlan / container dot1q-vlan

The descriptions talk about "matching frames" and "classifying
traffic", but it is not described anywhere how the matching and
classifying is used.

(also applies to ietf-flexible-encapsulation)

[rgwilton]

I've added the following paragraph to both models:

        "Flexibly classifies Ethernet frames to an interface or
         sub-interface based on the L2 header fields.

         Only frames matching the classification configured on an
         interface/sub-interface are processed on that
         interface/sub-interface.

         Frames that do not match any sub-interface are processed
         directly on the parent interface, if it is associated
         with a forwarding instance, otherwise they are dropped.

         If a frame could be classified to multiple
         sub-interfaces then they get classified to the
         sub-interface with the most specific match.  E.g.,
         matching two VLAN tags in the frame is more specific
         than matching the outermost VLAN tag, which is more
         specific than the catch all 'default' match.";

[samans]

Needs validation