Update documentation

Since recently (new gitlab release?), having just

script:
  - 'Adding tag'

fails with

Executing "step_script" stage of the job script 00:01
$ # INFO: Lowering limit of file descriptors for backwards compatibility. ffi: https://cern.ch/gitlab-runners-limit-file-descriptors # collapsed multi-line command
$ Adding tag...
/scripts-3588-32753363/step_script: eval: line 149: Adding: not found