When a user sets only_allow_merge_if_pipeline_succeeds, also named
as "Pipeline must succeed" project setting, we require the pipeline
to be present.

This solves race condition issues especially with external CI
providers when a build is triggered externally but no pipelines are
created in GitLab yet.

Document that a head pipeline is expected when using "Pipeline
must succeed" setting. Also explain limitations with the use of
only/except where there may not be any jobs created and the merge
request will not be allowed to be merged.