doc: explain how mirror zones use ACLs

Closes GL#375
10 jobs for u/fanf2/doc-mirror-acl in 10 minutes and 24 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Precheck
passed #22248
amd64 docker linux
precheck:debian:sid:amd64

00:00:50

 
  Build
passed #22249
amd64 docker linux
build:clang:debian:sid:amd64

00:04:19

passed #22251
i386 docker linux
build:clang:debian:sid:i386

00:04:37

passed #22250
amd64 docker linux
build:debian:sid:amd64

00:04:31

passed #22252
i386 docker linux
build:debian:sid:i386

00:04:48

 
  Test
passed #22256
amd64 docker linux
systemtest:debian:sid:amd64

00:04:44

passed #22257
i386 docker linux
systemtest:debian:sid:i386

00:04:43

passed #22254
amd64 docker linux
unittest:clang:debian:sid:amd64

00:00:44

passed #22253
amd64 docker linux
unittest:debian:sid:amd64

00:00:44

passed #22255
i386 docker linux
unittest:debian:sid:i386

00:00:47