[css-sizing-3] Drop stray reference to stretch keyword, mention origins of box-sizing and min-*:auto

Author: fantasai

css-sizing-3/Overview.bs

@@ -1358,9 +1358,9 @@ Additions since CSS Level 2</h3>
 	to the various automatic and content-based sizing algorithms,
 	the following new features have been added since [[CSS2]]:
 	<ul>
-		<li>The 'box-sizing' property
-		<li>The ''width/min-content'', ''width/max-content'', ''width/stretch'', and ''width/fit-content()'' values of the [=sizing properties=].
-		<li>The ''min-width/auto'' initial value of the 'min-width' and 'min-height' properties.
+		<li>The 'box-sizing' property (originally defined in [[CSS-UI-3]], then moved here).
+		<li>The ''width/min-content'', ''width/max-content'', and ''width/fit-content()'' values of the [=sizing properties=].
+		<li>The ''min-width/auto'' initial value of the 'min-width' and 'min-height' properties (originally defined in [[CSS-FLEXBOX-1]], then moved here).
 	</ul>
 
 <h2 class=no-num id="acknowledgments">