raddb: Comment on ipaddr/ipv4addr/ipv6addr use

I've attempted to summarize combined ipaddr/ipv4addr/ipv6addr use in raddb/sitest-available/default.
However, I might have got it wrong, sorry.

Also, our QE noticed that the following ChangeLog entry:

    * "ipaddr" will now use v6 if no v4 address is present.  You should
      use "ipv4addr" or "ipv6addr" to force v4/v6 addresses.

doesn't seem to be true WRT "ipaddr" v6 usage. It doesn't try to use IPv6, if there is no IPv4, it just aborts with "ip_hton: No address associated with hostname". I'm not sure if this is intended, and if not I'm not sure how to fix it. The comments attempt to describe the current state of affairs, but I'm ready to fix it if the behavior changes.

Thank you.