Improve the documentation on the NOFOLLOW mode in the dns_resolver mode

The following discussion from !6267 (merged) should be addressed:

• @ondrej started a discussion: (+3 comments)

  I think the commit message for this could do with a little bit more explanation. The code in resolver is hard to follow as is - an the result from rctx_answer() actually propagates down below to rctx_done() where it ends up in fctx_done_detach/fctx__done. But that still doesn't really explain where the DNS_R_DELEGATION is consumed in the end. Is that in ns_query or anywhere else?

  It's also not helpful that the code in resquery_response() has inverted logic.

  And furthermore DNS_FETCHOPT_NOFOLLOW is very poorly explained (the /*%< Don't follow delegations */ simply doesn't do it). Could we take this as an opportunity to actually explain somewhere in the code in more words what it does and why it is needed?