docs: add missing changelogs for 9939

changes/ce/fix-9939.en.md

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+Allow 'emqx ctl cluster' command to be issued before Mnesia starts.
+Prior to this change, EMQX `replicant` could not use `manual` discovery strategy.
+Now it's possible to join cluster using 'manual' strategy.

changes/ce/fix-9939.zh.md

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+允许 'emqx ctl cluster join' 命令在 Mnesia 启动前就可以调用。
+在此修复前， EMQX 的 `replicant` 类型节点无法使用 `manual` 集群发现策略。