Improve accessibility for for comment form

- Use a shorter label
- Remove now superfluous placeholder
- Use the old label as field description
#520

Author: MatzeKitt

templates/webmention-comment-form.php

@@ -5,11 +5,12 @@
 do_action( 'webmention_comment_form_template_before' );
 ?>
 <form id="webmention-form" action="<?php echo get_webmention_endpoint(); ?>" method="post">
-	<p>
-		<label for="webmention-source"><?php echo get_webmention_form_text( get_the_ID() ); ?></label>
+	<p id="webmention-source-description">
+		<?php echo get_webmention_form_text( get_the_ID() ); ?>
 	</p>
 	<p>
-		<input id="webmention-source" type="url" autocomplete="url" required pattern="^https?:\/\/(.*)" name="source" placeholder="<?php esc_attr_e( 'URL/Permalink of your article', 'webmention' ); ?>" />
+		<label for="webmention-source"><?php esc_attr_e( 'URL/Permalink of your article', 'webmention' ); ?></label>
+		<input id="webmention-source" type="url" autocomplete="url" required pattern="^https?:\/\/(.*)" name="source" aria-describedby="webmention-source-description" />
 	</p>
 	<p>
 		<input id="webmention-submit" type="submit" name="submit" value="<?php esc_attr_e( 'Ping me!', 'webmention' ); ?>" />