[css-scroll-anchoring-1] Move and expand section on anchor node invalidation

This commit moves the existing paragraph about anchor node invalidation into
a subsection, and adds precise conditions upon when an anchor node is considered
invalidated.

Author: eqrion

css-scroll-anchoring-1/Overview.bs

@@ -114,11 +114,6 @@ In some cases, a scrolling box may not select any anchor node.
 		content to shift without triggering any scroll anchoring adjustment.
 </div>
 
-Conceptually, a new anchor node is computed for every scrolling box
-whenever the scroll position of any scrolling box changes.
-(As a performance optimization,
-the implementation may wait until the anchor node is needed before computing it.)
-
 <div algorithm>
 	A DOM node |N| is an <dfn>excluded subtree</dfn>
 	if it is an element and any of the following conditions holds:
@@ -152,6 +147,18 @@ the implementation may wait until the anchor node is needed before computing it.
 	or the bounding rect of its <a>line boxes</a> if |N| is a text node.
 </div>
 
+<h4 id='anchor-node-invalidation'>
+Invalidation</h3>
+
+An anchor node is considered valid for a <a>scrolling box</a> until any of the following occur:
+	* The scroll position of the <a>scrolling box</a> changes (excluding adjustments originating from scroll anchoring).
+	* A <a>suppression trigger</a> is encountered.
+	* The <a>anchor node</a> is removed from the DOM.
+
+Once an anchor node has become invalid, the <a>anchor node selection algorithm</a> must be run again to compute the anchor node
+
+As a performance optimization, implementations may wait until the <a>anchor node</a> is needed before computing it.
+
 <h3 id="scroll-adjustment">
 Scroll Adjustment</h3>