Clarify comment about URL metric groups not changing across a single page load.

Co-authored-by: Weston Ruter <[email protected]>

Author: felixarntz

plugins/image-prioritizer/class-image-prioritizer-background-image-styled-tag-visitor.php

@@ -141,6 +141,7 @@ private function get_common_lcp_element_external_background_image( OD_URL_Metric
 	 */
 	private function maybe_preload_external_lcp_background_image( OD_Tag_Visitor_Context $context ): void {
 		// Gather the tuples of URL Metric group and the common LCP element external background image.
+		// Note the groups of URL Metrics do not change across invocations, we just need to compute this once for all.
 		if ( ! is_array( $this->group_common_lcp_element_external_background_images ) ) {
 			$this->group_common_lcp_element_external_background_images = array();
 			foreach ( $context->url_metric_group_collection as $group ) {