[css-nesting-1] Add an example explicitly showing a complex selector in the parent rule. Fixes w3c#7969

Author: tabatkins

css-nesting-1/Overview.bs

@@ -396,6 +396,12 @@ Syntax {#syntax}
 			:is(.error, #404):hover > .baz { color: red; }
 		*/
 
+		.ancestor .el {
+			.other-ancestor & { color: red; }
+		}
+		/* equivalent to
+			.other-ancestor :is(.ancestor .el) { color: red; }
+
 		<b>/* As can the nested selector */</b>
 		.foo {
 			& :is(.bar, &.baz) { color: red; }