[css-images-4][editorial] split Privacy & Security

Author: svgeesus

css-images-4/Overview.bs

@@ -2437,7 +2437,12 @@ as a parse-time alias of ''image-set()''.
 (It's a valid value, with identical arguments to ''image-set()'',
 and is turned into ''image-set()'' during parsing.)
 
-Privacy and Security Considerations {#priv-sec}
+Privacy Considerations {#privacy}
+==============================================
+
+Note: No change from [[css3-images]].
+
+Security Considerations {#security}
 ==============================================
 
 Note: No change from [[css3-images]].