Document macs not_valid_before and not_valid_after properties
Not valid before: when a mac address is assigned to an NSC, and the current datetime is before this value, then the MAC address cannot be used on the peering platform. Afterwards, it is supposed to be available. If the value is null or the property does not exist, the mac address is valid from the creation date.
Not valid after: when a mac address is assigned to an NSC, and the current datetime is before this value, the MAC address can be used on the peering platform. Afterwards, it is supposed to be unassigned from the NSC and cannot any longer be used on the peering platform. If the value is null or the property does not exist, the MAC address is indefinitely. The may value may not be in the past.
Note: setting the above two parameters supports maintenance windows in which the MAC addresses overlap for certain period of time. The implementation of the maintenance windows is dependent on the specific exchange. While some IXPs allow them, the business of other IXPs may reject them. If both values are set, this value needs to be bigger than "Not valid before".