Docs: Migration guide CI -> CE/EE bugs

Backup task

The command for creating backups of the CI installation is

sudo -u gitlab_ci -H bundle exec rake backup:create RAILS_ENV=production

The documentary is missing the rake command which leads to an error message:

bundler: command not found: backup:create

Title changed from Docs: Migration guide CI -> CE/EE bug to Docs: Migration guide CI -> CE/EE bugs

/cc @rspeicher

Reassigned to @rspeicher

Milestone changed to 8.0

Reassigned to @marin

In gitlab-org/gitlab-ce!1385

Merged and added to stable branches. Thanks @frostieDE

Status changed to closed