gitlab: mention 2fa problems in the restore procedure (team#40820)

howto/gitlab.md

@@ -796,6 +796,11 @@ root):
     for a dev server, because you do not want a possible dev server
     compromise to escalate to the production server, or the dev server
     to have access to the prod deployments.
+    
+    Also note that this step was *not* performed on the dev server
+    test and this lead to problems during login: while it was possible
+    to use a recovery code to bypass 2FA, it wasn't possible to reset
+    the 2FA configuration afterwards.
 
  6. restore the files: