doc: Explicit more APT persistence.

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

@@ -169,8 +169,9 @@ When this feature is activated, the packages that you install using the
 <span class="application">Synaptic</span> package manager or the
 <span class="command">apt-get</span> command are saved to the persistent storage.
 
-Note that those packages are not automatically reinstalled when starting a new
-Tails working session.
+If you install additional programs, this feature allows you to download them
+once and reinstall them during future working sessions, even offline.
+Note that those packages are not automatically installed when restarting Tails.
 
 If you activate this feature, it is recommended to activate the
 <span class="guilabel">APT Lists</span> feature as well.
@@ -189,6 +190,10 @@ downloaded while doing
 <span class="application">Synaptic</span> package manager or issuing the
 <span class="command">apt-get update</span> command.
 
+The <span class="emphasis">APT lists</span> are needed to install additional
+programs or explore the list of available software packages. This feature allows
+you to reuse them during future working sessions, even offline.
+
 <div class="icon">
 <div class="image"><img src="../preferences-desktop.png"/></div>
 <div class="text"><h2>Dotfiles</h2></div>