docs(window-sync): add zen-sessions.jsonzl4 check in "Updating" section (#257)

Author: KiKaraage

content/docs/user-manual/window-sync.mdx

@@ -48,6 +48,8 @@ Unlike previous implementation where Essentials and pinned tabs were duplicated
 
 - Open your current Zen profile directory.
 - **Make sure Zen is fully closed**.
+- In the profile directory, rename `zen-sessions.jsonlz4` to **zen-sessions.jsonlz4.bak**.
+- If there is `sessionstore.jsonlz4` in the directory, rename it to **sessionstore.jsonlz4.bak** too.
 
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