Document Delete Merge Branches won't touch protected branches

Summary

"Delete merged branches" is not working

Steps to reproduce

Click "Delete merged branches", then show message "Merged branches are being deleted. This can take some time depending on the number of branches. Please refresh the page to see changes.". But after a long time (several days), the branches with label "merged" were still there.

Example Project

https://gitlab.com/siceberg/testGitlabFlow/

Output of checks

This bug happens on GitLab.com