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 precheck:debian:sid:amd64 #22129
amd64 docker linux

00:01:10

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

00:04:25

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

00:05:55

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

00:04:46

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

00:05:53

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

00:05:49

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

00:06:04

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

00:00:47

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

00:00:46

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

00:00:48