[css-conditional-3][editorial] preferred text for no security issues

Author: svgeesus

css-conditional-3/Overview.bs

@@ -963,7 +963,7 @@ js/CSS-supports-L3.html
 
 <h2 class=no-num id=security>Security Considerations</h2>
 
-This spec introduces no new security considerations.
+No security issues have been raised against this document
 
 <h2 class=no-num id=privacy>Privacy Considerations</h2>