Explain when these instructions apply

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

@@ -2,7 +2,8 @@
 
 <!-- XXX: These instructions should be kept in sync with doc/first_steps/persistence/backup.mdwn -->
 
-<!-- TODO: Explain when these instructions apply. -->
+If your Tails becomes broken, for example, it no longer starts, you can follow
+these instructions to try to rescue your Persistent Storage.
 
 1. Install Tails on a new USB stick on which you want to rescue your Persistent
    Storage. Make sure that this new USB stick is at least