Document DHCPv6 options set by Kea
There's a long list of v6 options that Kea supports, but they're not listed as supported, because they can't be configured explicitly. However, support requested a documentation for similar options in v4 (see #1323 (closed)) and @tomek added it. We should have a similar list for v6.
It's actually easy to do: checkout old branch (e.g. v1_4_0) and look at doc/guide/dhcp6-srv.xml for comments around line 1344.