[css-display] Clarify that box fixup is after display: none/contents, and that these values do not affect markup-based relationships.

Author: fantasai

css-display/Overview.bs

@@ -487,6 +487,17 @@ Box Generation: the ''none'' and ''contents'' keywords</h3>
 	Elements with either of these values do not have <a>inner</a> or <a>outer display types</a>,
 	because they don't generate any boxes at all.
 
+	Note: As these values cause affected elements to not generate a box,
+	anonymous box generation rules will ignore the elided elements entirely,
+	as if they did not exist in the box tree.
+
+	Markup-based relationships, however, are not affected by these values,
+	as they are solely rendering-time effects.
+	For example,
+	although they may affect which table cell <em>appears</em> in a column,
+	they do not affect which table cell is associated with a particular column <em>element</em>.
+	Similarly, they cannot affect which HTML <{summary}> element is associated with a particular table or whether a <{legend}> is the name of a <{fieldset}>.
+
 <h3 id='legacy-display'>
 Precomposed Inline-level Display Values</h3>