[css-writing-modes] Add comment pointing to orthogonal flow scroller contstraint discussion.

Author: fantasai

css-writing-modes-3/Overview.bs

@@ -2035,6 +2035,7 @@ Available Space of Orthogonal Flows</h4>
       <li>the nearest ancestor <a>scrollport</a>’s inner size if that is fixed,
           else / capped by its inner <a>max size</a> if that is fixed,
           floored by its inner <a>min size</a> if that is fixed
+          <!-- https://lists.w3.org/Archives/Public/www-style/2014Dec/0301.html -->
       <li>the initial containing block’s size
     </ul>
 

css-writing-modes-4/Overview.bs

@@ -2127,6 +2127,7 @@ Available Space in Orthogonal Flows</h4>
       <li>the nearest ancestor <a>scrollport</a>’s inner size if that is fixed,
           else / capped by its inner <a>max size</a> if that is fixed,
           floored by its inner <a>min size</a> if that is fixed
+          <!-- https://lists.w3.org/Archives/Public/www-style/2014Dec/0301.html -->
       <li>the initial containing block’s size
     </ul>