Update image-prioritizer test case

Author: westonruter

plugins/image-prioritizer/tests/test-cases/common-lcp-image-with-old-xpath-format/expected.html

@@ -3,7 +3,8 @@
 	$test_case->populate_url_metrics(
 		array(
 			array(
-				// Note: This is intentionally using old XPath scheme. This is to make sure that the old format still results in the expected optimization during a transitional period.
+				// Note: This is intentionally using old XPath scheme to test the behavior in case an old URL Metric ends up getting used.
+				// In practice, this should behave identically to the common-lcp-image-with-stale-sample-data test.
 				'xpath' => '/*[1][self::HTML]/*[2][self::BODY]/*[1][self::DIV]/*[1][self::IMG]',
 				'isLCP' => true,
 			),