CP-312160: update design doc according to review comments

Signed-off-by: Chunjie Zhu <chunjie.zhu@cloud.com>

Author: chunjiez

doc/content/design/secureboot-certificate-expiry.md

@@ -44,6 +44,7 @@ stateDiagram
 update_available --> update_on_boot : Admin marks VM for update
 update_on_boot --> ok : VM boots, update succeeds
 update_on_boot --> update_on_boot : VM boots, update fails(retain state)
+ok --> update_available : recompute state(e.g. legacy VM import)
 
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