[css-grid] Fill out some details on the orthogonal flows issue.

Author: fantasai

css-grid/Overview.bs

@@ -2632,6 +2632,8 @@ Overall Sizing Algorithm</h3>
 		<li>
 			First, the <a>track sizing algorithm</a> is used to resolve the sizes of the grid columns.
 
+			Issue: Need to assign preliminary row sizes in order to handle orthogonal flows.
+
 		<li>
 			Next, the <a>track sizing algorithm</a> resolves the sizes of the grid rows.
 
@@ -2640,8 +2642,15 @@ Overall Sizing Algorithm</h3>
 			based on the row sizes calculated in step 2,
 			steps 1 and 2 are repeated with the new <a>min-content contribution</a> (once only).
 
-			<p class='issue'>
-				What is this solving and is this the right solution?
+			<div class="note">
+				This cycle is necessary for cases where the <a>inline size</a> of a <a>grid item</a>
+				depends on the <a>block size</a> of its <a>grid area</a>.
+				Examples include wrapped column <a>flex containers</a> (''flex-flow: column wrap''),
+				<a>orthogonal flows</a> ('writing-mode'),
+				and <a spec=css-multicol>multi-column element</a>.
+			</div>
+
+			Issue: Is one cycle enough?
 	</ol>
 
 	Once the size of each <a>grid area</a> is thus established,