[css-multicol] Split security and privacy sections.

Author: rachelandrew

css-multicol-1/Overview.bs

@@ -1643,10 +1643,13 @@ This appendix is <em>informative</em>.
 	<li>Content that overflows columns is not clipped.
 </ul>
 
-<h2 class="no-num" id="priv-sec">Privacy and Security Considerations</h2>
+<h2 class="no-num" id="privacy">Privacy Considerations</h2>
 
-Multicol introduces no new privacy leaks,
-or security considerations beyond "implement it correctly".
+Multicol introduces no new privacy leaks.
+
+<h2 class="no-num" id="security">Security Considerations</h2>
+
+Multicol introduces no new security considerations.
 
 <h2 class="no-num" id="a11y">Accessibility Considerations</h2>