Add a comment explaining a mirror zone glitch

Explain why in a certain edge case mirror zone data may not be used for
resolution purposes despite being available.
39 jobs for !1480 with 870-add-a-comment-explaining-a-mirror-zone-glitch in 12 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Precheck
canceled #172821
amd64 linux
🐞:sid:amd64

00:00:12

canceled #172820
amd64 linux
misc:sid:amd64

00:00:12

 
  Build
canceled #172832
amd64 linux
asan:sid:amd64

canceled #172833
amd64 linux
clang:stretch:amd64

canceled #172834
i386 linux
clang:stretch:i386

canceled #172822
amd64 linux
docs:sid:amd64

canceled #172831
amd64 linux
gcc:bionic:amd64

canceled #172823
amd64 linux
gcc:centos6:amd64

canceled #172824
amd64 linux
gcc:centos7:amd64

canceled #172829
amd64 linux
gcc:fedora29:amd64

canceled #172825
amd64 linux
gcc:jessie:amd64

canceled #172827
amd64 linux
gcc:sid:amd64

canceled #172828
i386 linux
gcc:sid:i386

canceled #172826
amd64 linux
gcc:stretch:amd64

canceled #172830
amd64 linux
gcc:xenial:amd64

canceled #172835
amd64 linux
pkcs11:sid:amd64

 
  Test
canceled #172854
amd64 linux
system:asan:sid:amd64

canceled #172852
amd64 linux
system:gcc:bionic:amd64

canceled #172836
amd64 linux
system:gcc:centos6:amd64

canceled #172838
amd64 linux
system:gcc:centos7:amd64

canceled #172848
amd64 linux
system:gcc:fedora29:amd64

canceled #172840
amd64 linux
system:gcc:jessie:amd64

canceled #172844
amd64 linux
system:gcc:sid:amd64

canceled #172846
i386 linux
system:gcc:sid:i386

canceled #172842
amd64 linux
system:gcc:stretch:amd64

canceled #172850
amd64 linux
system:gcc:xenial:amd64

canceled #172857
amd64 linux
system:pkcs11:sid:amd64

canceled #172855
amd64 linux
unit:asan:sid:amd64

canceled #172856
amd64 linux
unit:clang:stretch:amd64

canceled #172853
amd64 linux
unit:gcc:bionic:amd64

canceled #172837
amd64 linux
unit:gcc:centos6:amd64

canceled #172839
amd64 linux
unit:gcc:centos7:amd64

canceled #172849
amd64 linux
unit:gcc:fedora29:amd64

canceled #172841
amd64 linux
unit:gcc:jessie:amd64

canceled #172845
amd64 linux
unit:gcc:sid:amd64

canceled #172847
i386 linux
unit:gcc:sid:i386

canceled #172843
amd64 linux
unit:gcc:stretch:amd64

canceled #172851
amd64 linux
unit:gcc:xenial:amd64

canceled #172858
amd64 linux
unit:pkcs11:sid:amd64