I18N: Mark screen reader strings as such with translator comments.

This aims to provide better context for translators and make it easier to determine that some strings contain hidden accessibility text and are not displayed in the UI.

Props kebbet, mercime, pavelevap, ocean90, swissspidy, Chouby, jipmoors, afercia, desrosj, costdev, audrasjb, SergeyBiryukov.
Fixes #29748.

git-svn-id: https://develop.svn.wordpress.org/trunk@55276 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/widgets-form.php

@@ -423,7 +423,12 @@ do_action( 'widgets_admin_page' );
 	<div id="available-widgets" class="widgets-holder-wrap">
 		<div class="sidebar-name">
 			<button type="button" class="handlediv hide-if-no-js" aria-expanded="true">
-				<span class="screen-reader-text"><?php _e( 'Available Widgets' ); ?></span>
+				<span class="screen-reader-text">
+					<?php
+					/* translators: Hidden accessibility text. */
+					_e( 'Available Widgets' );
+					?>
+				</span>
 				<span class="toggle-indicator" aria-hidden="true"></span>
 			</button>
 			<h2><?php _e( 'Available Widgets' ); ?> <span id="removing-widget"><?php _ex( 'Deactivate', 'removing-widget' ); ?> <span></span></span></h2>