[css-flexbox-1] Mention aspect ratio as a case where the main size depends on the cross size. #6109

Author: fantasai

css-flexbox-1/Overview.bs

@@ -2588,7 +2588,8 @@ Line Length Determination</h3>
 					using its used <a>flex basis</a> in place of its <a>main size</a>,
 					treating a value of ''content'' as ''width/max-content''.
 					If a <a>cross size</a> is needed to determine the <a>main size</a>
-					(e.g. when the <a>flex item</a>’s <a>main size</a> is in its block axis)
+					(e.g. when the <a>flex item</a>’s <a>main size</a> is in its block axis,
+					or when it has a [=preferred aspect ratio=])
 					and the <a>flex item</a>’s cross size is ''auto'' and not <a>definite</a>,
 					in this calculation use ''width/fit-content'' as the <a>flex item</a>’s <a>cross size</a>.
 					The <a>flex base size</a> is the item's resulting <a>main size</a>.