Document the interation of DNS COOKIE and RRL

Send back BADCOOKIE responses instead of TC=1 when slipping.

Skip rate limiting for UDP requests with valid server cookies.

(cherry picked from commit a59482b8)
72 jobs for !5701 with 3025-document-how-rate-limiting-uses-dns-cookies-v9_16 in 19 minutes and 6 seconds (queued for 15 seconds)
latest merge request