[css-sizing] Add issue wrt margin collapsing on 'stretch'.

Author: fantasai

css-sizing-3/Overview.bs

@@ -420,6 +420,8 @@ Stretch-fit Sizing</h3>
 		(after any <a href="https://www.w3.org/TR/CSS21/box.html#collapsing-margins">margin collapsing</a>, and treating ''margin/auto'' margins as zero),
 		borders, and padding.
 
+		Issue: Shouldn't this be ignoring margin collapsing?
+
 	<p>
 		The <a>stretch-fit block size</a> of a box is defined analogously,
 		but in the other dimension.

css-sizing/Overview.bs

@@ -580,6 +580,8 @@ Stretch-fit Sizing</h3>
 		(after any <a href="https://www.w3.org/TR/CSS21/box.html#collapsing-margins">margin collapsing</a>, and treating ''margin/auto'' margins as zero),
 		borders, and padding.
 
+		Issue: Shouldn't this be ignoring margin collapsing?
+
 	<p>
 		The <a>stretch-fit block size</a> of a box is defined analogously,
 		but in the other dimension.