[css-sizing-4] Remove the introductory issue about a-r being a rough draft. Replace it with an intro paragraph.

Author: tabatkins

css-sizing-4/Overview.bs

@@ -129,9 +129,13 @@ New Sizing Values: the ''stretch'', ''fit-content'', and ''contain'' keywords</h
 <h2 id="ratios">
 Aspect Ratios</h2>
 
-ISSUE: This section is a rough draft proposal.
-See discussion in <a href="https://github.com/w3c/csswg-drafts/issues/333">Issue 333</a>
-and <a href="https://github.com/w3c/csswg-drafts/issues/1173">Issue 1173</a>.
+	Images often have an [=intrinsic aspect ratio=],
+	which the CSS layout algorithms attempt to preserve
+	as they resize the element.
+
+	The 'aspect-ratio' property allows specifying this behavior
+	for non-replaced elements,
+	and for altering the effective aspect ratio of replaced elements.
 
 <h3 id="aspect-ratio">
 Intrinsic Aspect Ratios: the 'aspect-ratio' property</h3>