[css-sizing-4][editorial] Unmerge the privsec sections, per W3C policy

Author: tabatkins

css-sizing-4/Overview.bs

@@ -1060,6 +1060,11 @@ Acknowledgments</h2>
 	for their contributions to this module.
 
 <h2 class=no-num id=priv-sec>
-Privacy and Security Considerations</h2>
+Privacy Considerations</h2>
 
-This specification introduces no new privacy or security considerations.
+This specification introduces no new privacy considerations.
+
+<h2 class=no-num id=sec>
+Security Considerations</h2>
+
+This specification introduces no new security considerations.