Improve contain-intrinsic-size description (#843)

As suggested by @vmpstr. Thanks!

Author: foolip

feature-group-definitions/contain-intrinsic-size.yml

@@ -1,5 +1,5 @@
 name: contain-intrinsic-size
-description: The `contain-intrinsic-size` CSS property sets the size of an element that the browser will use for layout calculations when the element is subject to size containment, within a CSS query container.
+description: The `contain-intrinsic-size` CSS property sets the intrinsic size of an element. When using size containment, the browser will layout the element as if it had a single child of this size.
 spec: https://drafts.csswg.org/css-sizing-4/#intrinsic-size-override
 # Shown as "intrinsic-size" on chromestatus.com, but 651 is contain-intrinsic-size.
 usage_stats: https://chromestatus.com/metrics/css/timeline/popularity/651