doc: document TLS_FORWARD issues with some public DNS providers

27 jobs for <span class="ref-name">doc-google-public-dns</span> in 7 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
canceled #282286
docker linux amd64
archive

00:00:07

canceled #282287
docker linux amd64
build

00:00:07

canceled #282288
docker linux amd64
build-asan

00:00:07

canceled #282289
docker
kres-gen

00:00:07

 
  Test
canceled #282290
docker delayed
build:darwin

canceled #282291
docker linux amd64
deckard

canceled #282292
docker delayed
doc

canceled #282293
dind
docker

canceled #282294
docker delayed
lint:other

canceled #282295
docker linux amd64 delayed
lint:pedantic

canceled #282296
docker linux amd64
lint:scan-build

canceled #282297
docker linux amd64
lint:tidy

canceled #282298
privileged
pkg:epel-7

canceled #282299
privileged
pkg:fedora-29

canceled #282300
docker linux amd64
respdiff:basic

canceled #282301
docker linux amd64 delayed
test:valgrind

 
  Extended
canceled #282303
respdiff
fwd-tls6-kresd.udp6

canceled #282304
respdiff
fwd-udp6-kresd.udp6

canceled #282308
respdiff
fwd-udp6-unbound.tcp6

canceled #282309
respdiff
fwd-udp6-unbound.tls6

canceled #282307
respdiff
fwd-udp6-unbound.udp6

canceled #282306
respdiff
iter.tls6

canceled #282305
respdiff
iter.udp6

canceled #282302
docker linux amd64
pytests

canceled #282310
respdiff
resperf:fwd-tls6.udp-asan

canceled #282311
respdiff
resperf:fwd-udp6.udp-asan

canceled #282312
respdiff
resperf:iter.udp-asan