clarify documentation of the 'mirror' option

10 jobs for clarify-mirror in 13 minutes and 18 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Precheck
passed #22129
amd64 docker linux
precheck:debian:sid:amd64

00:01:10

 
  Build
passed #22130
amd64 docker linux
build:clang:debian:sid:amd64

00:04:25

passed #22132
i386 docker linux
build:clang:debian:sid:i386

00:05:55

passed #22131
amd64 docker linux
build:debian:sid:amd64

00:04:46

passed #22133
i386 docker linux
build:debian:sid:i386

00:05:53

 
  Test
passed #22137
amd64 docker linux
systemtest:debian:sid:amd64

00:05:49

passed #22138
i386 docker linux
systemtest:debian:sid:i386

00:06:04

passed #22135
amd64 docker linux
unittest:clang:debian:sid:amd64

00:00:47

passed #22134
amd64 docker linux
unittest:debian:sid:amd64

00:00:46

passed #22136
i386 docker linux
unittest:debian:sid:i386

00:00:48