Replace RADIUS attribute Framed-IPv6-Pool with Framed-Pool in the documentation

The ARM mentions Framed-IPv6-Pool: In this case, the RADIUS server sends back an Access-Accept with a Framed-Pool (IPv4) or Framed-IPv6-Pool (IPv6).. However v6 works with Framed-Pool instead.

1. Configure a Framed-IPv6-Pool in freeradius e.g.

00:03:00:01:08:00:27:b0:c1:42    Cleartext-password := "08:00:27:b0:c1:42"
    Framed-IPv6-Pool = "gold"

2. Start kea-dhcp6 with RADIUS configured and a "gold" classified pool e.g.

"pools": [
  {
    "client-class": "gold",
    "pool": "2001:db8:50::5 - 2001:db8:50::5"
  }
]

3. Send a solicit.
4. Observe the NoAddrsAvail.
5. Change Framed-IPv6-Pool to Framed-Pool and restart RADIUS and Kea.
6. Send solicit again.
7. Observe that it gets a lease from the pool.

I think PW_FRAMED_IPV6_POOL needs to be used in lines: radius_access:766, radius_callout.cc:382.

Edit: After Francis pointed out that Framed-Pool should be used for plain addresses in v6 according to the RFC, it became clear that the change should be in the docs.