add warning and example of fake card and repair

I just tested those instructions and successfully repaired a fake card
this way. There were extra steps needed for the real size to be taken
into account.

It also wasn't very clear that `f3probe` would destroy data, so add a
very explicit warning about this.
1 job for master in 5 minutes and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
failed #39981601


Name Stage Failure
build Test There is an unknown failure, please try again
WARNING: grml32/grml_logs/: no matching files      
WARNING: grml64/grml_isos/: no matching files
WARNING: grml64/netboot/: no matching files
grml64/grml_logs/: found 18 matching files
WARNING: grml96/grml_isos/: no matching files
WARNING: grml96/netboot/: no matching files
WARNING: grml96/grml_logs/: no matching files
Uploading artifacts to coordinator... ok id=39981601 responseStatus=201 Created token=R2WbcK1c
ERROR: Job failed: exit code 1