odhcpd: document the ubus interface

odhcpd: document the ubus interface

I guess this might be subject to change (for example, we probably want to
also broadcast events for DHCPv6 leases in the future, and I guess the
ack/release/expire events that are currently broadcast might benefit from
being renamed to something that makes it clearer that they are DHCPv4
specific events), but this is the current ubus interface.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/270
Signed-off-by: Álvaro Fernández Rojas <[email protected]>