Add doc block for detect.js

plugins/embed-optimizer/detect.js

@@ -1,3 +1,11 @@
+/**
+ * Embed Optimizer module for Optimization Detective
+ *
+ * When a URL metric is being collected by Optimization Detective, this module adds a ResizeObserver to keep track of
+ * the changed heights for embed blocks. This data is amended onto the element data of the pending URL metric when it
+ * is submitted for storage.
+ */
+
 const consoleLogPrefix = '[Embed Optimizer]';
 
 /**