Add notes to keep instructions in sync

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wiki/src/doc/first_steps/persistence/backup.mdwn

 [[!meta title="Making a backup of your Persistent Storage"]]
 
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 These instructions explain how to back up your Persistent Storage to
 another Tails USB stick.
 

wiki/src/doc/first_steps/persistence/rescue.mdwn

 [[!meta title="Rescuing your Persistent Storage from a broken Tails"]]
 
+<!-- XXX: These instructions should be kept in sync with doc/first_steps/persistence/backup.mdwn -->
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 <!-- TODO: Explain when these instructions apply. -->
 
 1. Install Tails on a new USB stick on which you want to rescue your Persistent