Be affirmative (#7076)

This sometimes make sentences shorter (like in this first sentence) or
sometimes removes ambiguity (like in this second sentence with 'can').

wiki/src/doc/about/warning.mdwn

@@ -15,9 +15,9 @@ Tails does not protect against compromised hardware, BIOS or firmware attacks
 
 If the computer has been compromised by someone having physical access
 to it and who installed untrusted pieces of hardware (like a
-keylogger), then it might not be safe to use Tails.
+keylogger), then it might unsafe to use Tails.
 
-Tails can also not protect against BIOS or firmware attacks. Those are
+It is also impossible for Tails to protect against BIOS or firmware attacks. Those are
 not managed by the operating system, and in practice they function
 like hardware.