NIC Tag documentation improvement

[aboron]

While attempting to add a 3rd physical network I consulted the documentation on creating NIC Tags at:
https://docs.joyent.com/private-cloud/networks/nic-tags
And also the creating network definition page at:
https://docs.joyent.com/private-cloud/networks

Both were complete with many screen shots and details on how to create a network and a nic tag, but neither page goes all the way to showing where to apply the nic tag to a physical nic on the servers page.

The installation does this automatically for admin and external networks, so a brand new user has no way of seeing the full process.

The documentation might benefit from this addition, and some related notes about using a nic tag with a network in order to associate it to the physical server hardware.