If a new L2 connection was established, the node should be inserted in
the neighbor cache.
For 6LN nodes, NIB will never start the usual neighbor discovery process
because it can directly resolve the link-layer address from the IPv6
address. Thus we have to manually add such nodes to the NC by building
the IPv6 address from the L2 address.
If a L2 connection closed, the node is removed again from the NC.
I applied the following terminology and changed the wording in the doc
accordingly:
* must not: If the parameter is of the value it *must not* be it either
hits an assert or crashes the system.
* may not: The value can be that value, but the function will return an
error.
