doc: Fix warning about secure deletion of the persistent storage.

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

@@ -13,12 +13,16 @@ This can be useful in order to delete all the files saved to the persistent
 storage in a single operation. You can later create a new persistent storage on
 the same USB stick without having to reinstall Tails.
 
-Creating a new persistent storage on top of an old one makes it hard for an
-attacker to recover the files of the old persistent storage using [[data
-recovery techniques|encryption_and_privacy/secure_deletion#index1h1]]. But for
-more security, you can do the following operations on the USB stick:
+<div class="caution">
+<strong>This technique may not prevent an attacker to recover the files of the
+old persistent storage using [[data
+recovery techniques|encryption_and_privacy/secure_deletion#index1h1]].</strong>
+For more security, you can do the following operations on the USB stick:
 
-1. [[Erase Tails|resetting_a_usb_stick]]
-2. [[Securely clean all the available disk space|encryption_and_privacy/secure_deletion#index4h1]]
-3. [[Reinstall Tails|usb_installer]]
-4. [[Create a new persistent storage|persistence/configure]]
+<ol>
+<li>[[Erase Tails|resetting_a_usb_stick]]</li>
+<li>[[Securely clean all the available disk space|encryption_and_privacy/secure_deletion#index4h1]]</li>
+<li>[[Reinstall Tails|usb_installer]]</li>
+<li>[[Create a new persistent storage|persistence/configure]]</li>
+</ol>
+</div>