E2E wpDataSelect: Explain why undefined is returned sometimes (#31227)

packages/e2e-test-utils/README.md

@@ -717,6 +717,19 @@ _Parameters_
 
 Queries the WordPress data module.
 
+`page.evaluate` - used in the function - returns `undefined`
+when it encounters a non-serializable value.
+Since we store many different values in the data module,
+you can end up with an `undefined` result. Before using
+this function, make sure the data you are querying
+doesn't contain non-serializable values, for example,
+functions, DOM element handles, etc.
+
+_Related_
+
+-   <https://pptr.dev/#?product=Puppeteer&version=v9.0.0&show=api-pageevaluatepagefunction-args>
+-   <https://github.com/WordPress/gutenberg/pull/31199>
+
 _Parameters_
 
 -   _store_ `string`: Store to query e.g: core/editor, core/blocks...

packages/e2e-test-utils/src/wp-data-select.js

@@ -1,6 +1,17 @@
 /**
  * Queries the WordPress data module.
  *
+ * `page.evaluate` - used in the function - returns `undefined`
+ * when it encounters a non-serializable value.
+ * Since we store many different values in the data module,
+ * you can end up with an `undefined` result. Before using
+ * this function, make sure the data you are querying
+ * doesn't contain non-serializable values, for example,
+ * functions, DOM element handles, etc.
+ *
+ * @see https://pptr.dev/#?product=Puppeteer&version=v9.0.0&show=api-pageevaluatepagefunction-args
+ * @see https://github.com/WordPress/gutenberg/pull/31199
+ *
  * @param {string}    store      Store to query e.g: core/editor, core/blocks...
  * @param {string}    selector   Selector to exectute e.g: getBlocks.
  * @param {...Object} parameters Parameters to pass to the selector.