document ability to disable MACAddressPolicy

document ability to disable MACAddressPolicy

While it is currently possible to either not set MACAddressPolicy or set
it to a value different from "persistent" or "random", it is not obvious
that a user can do so. Add a policy, "none", which simply retains kernel
MAC addresses (same as not filling in the policy at all) and document it
so that users are aware of this setting.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>