Add note about automatically re-enabling CI during migrate task

[ci skip]

doc/migrate_ci_to_ce/README.md

@@ -214,6 +214,9 @@ GitLab CE or EE database:
 This task will take some time.
+This migration task automatically re-enables the CI setting that you
+[disabled earlier](#2-prevent-ci-usage-during-the-migration-process).
+
 #### 9. Start GitLab
 You can start GitLab CE (or EE) now and see if everything is working: