[css-contain] Undo part of commit b677c01

The resolution in issue #3280 did not include a change in the grammar of
the property, so doing that in the commit was a mistake. Reverting that
bit, while keeping the (correct) changes in semantics.

Author: frivoal

css-contain-1/Overview.bs

@@ -61,7 +61,7 @@ Strong Containment: the 'contain' property</h2>
 
 	<pre class='propdef'>
 		Name: contain
-		Value: none | [strict | content | [ size || layout || paint ] ] || style
+		Value: none | strict | content | [ size || layout || style || paint ]
 		Initial: none
 		Inherited: no
 		Applies to: See <a href="#contain-applies">below</a>