Unexpected behavior during mirroring (push) protected branches.

Problem to solve

When a Gitlab mirror is set up to push protected branches only, Gitlab pushes all the branches if there are no protected branches in the repository.

We should have a note that mentions this behaviour in the documentation where push mirrors are described.

Link to Documentation Section: https://docs.gitlab.com/ee/user/project/repository/repository_mirroring.html#push-only-protected-branches-core

Proposal

Add a note in the relevant documentation section that says.

If the repository has no protected branches, then gitlab will push all branches to the mirror.