Updates the alt text description text in both the media attachment post screen and media manager templates.

   Updates the alt text description text in both the media attachment post screen and media manager templates. The new text is clearer and more concise, and includes an external link icon for better user experience.

   Trac ticket: https://core.trac.wordpress.org/ticket/64374

   ---
   **This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See [GitHub Pull Requests for Code Review](https://make.wordpress.org/core/handbook/contribute/git/github-pull-requests-for-code-review/) in the Core Handbook for more details.**

Author: burtrw

src/wp-admin/includes/media.php

@@ -3237,16 +3237,17 @@ function edit_form_image_editor( $post ) {
 		<?php
 
 		printf(
-			/* translators: 1: Link to tutorial, 2: Additional link attributes, 3: Accessibility text. */
-			__( '<a href="%1$s" %2$s>Learn how to describe the purpose of the image%3$s</a>. Leave empty if the image is purely decorative.' ),
+			/* translators: 1: Link to tutorial, 2: Additional link attributes, 3: Accessibility text, 4: External link icon. */
+			__( 'Alt text describes the image\'s purpose. Leave it blank if the image is purely decorative. <a href="%1$s" %2$s>Learn more about alt text%3$s%4$s</a>.' ),
 			/* translators: Localized tutorial, if one exists. W3C Web Accessibility Initiative link has list of existing translations. */
 			esc_url( __( 'https://www.w3.org/WAI/tutorials/images/decision-tree/' ) ),
 			'target="_blank"',
 			sprintf(
 				'<span class="screen-reader-text"> %s</span>',
 				/* translators: Hidden accessibility text. */
 				__( '(opens in a new tab)' )
-			)
+			),
+			'<span aria-hidden="true" class="dashicons dashicons-external"></span>'
 		);
 
 		?>

src/wp-includes/media-template.php

@@ -157,16 +157,17 @@ function wp_print_media_templates() {
 	$class = 'media-modal wp-core-ui';
 
 	$alt_text_description = sprintf(
-		/* translators: 1: Link to tutorial, 2: Additional link attributes, 3: Accessibility text. */
-		__( '<a href="%1$s" %2$s>Learn how to describe the purpose of the image%3$s</a>. Leave empty if the image is purely decorative.' ),
+		/* translators: 1: Link to tutorial, 2: Additional link attributes, 3: Accessibility text, 4: External link icon. */
+		__( 'Alt text describes the image\'s purpose. Leave it blank if the image is purely decorative. <a href="%1$s" %2$s>Learn more about alt text%3$s%4$s</a>.' ),
 		/* translators: Localized tutorial, if one exists. W3C Web Accessibility Initiative link has list of existing translations. */
 		esc_url( __( 'https://www.w3.org/WAI/tutorials/images/decision-tree/' ) ),
 		'target="_blank"',
 		sprintf(
 			'<span class="screen-reader-text"> %s</span>',
 			/* translators: Hidden accessibility text. */
 			__( '(opens in a new tab)' )
-		)
+		),
+		'<span aria-hidden="true" class="dashicons dashicons-external"></span>'
 	);
 	?>