how to schedule a pipeline that runs "only once"?

The pipeline schedule docs (here) state that

Pipeline schedules can be used to run pipelines only once, or for example every month on the 22nd for a certain branch.

(emphasis mine)

However, it's not clear how to make a schedule that runs "only once".

Go to "Pipeline Schedules" for a project and click on "New schedule".

Run "only once" doesn't seem to be implemented, despite what the docs say. Or if it is implemented, its quite buried and non-obvious.

The docs should match the implementation.

GitLab Community Edition 10.0.4

Implement "run once" or fix the docs.