Clarify what happens if both except and only are defined and are using the same definition
This came up in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11813#note_165001179. We should document that `except` takes a priority over `only` if both are defined and are using the same definition.
https://docs.gitlab.com/ce/ci/yaml/#onlyexcept-basic
issue