doc: Rewrite warning about installing additional software.

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

@@ -24,13 +24,14 @@ Furthermore, the anonymity of Tor and Tails relies on making it harder to
 distinguish one Tails user from another. <strong>Changing the default
 configurations can break your anonymity.</strong>
 
-Installing New Software
-=======================
-
-The programs included in Tails are selected with care and audited for their
-security. This may not be the case of all the packages available in Debian.
-**Other programs than the ones included in Tails may be subject to security
-holes that Tails cannot protect you from.**
+Installing Additional Programs
+==============================
+
+To protect your anonymity and leave no trace, Tails developers select and
+configure with care programs that work well together. **Installing additional
+programs may introduce unpredictable problems and may break the protections
+built-in Tails.** Tails developers may not want or may not be capable of helping
+you to solve those problems.
 
 Browser Plugins
 ===============