Customize: Add explanatory note for custom_css control via textarea[placeholder] and expose any defined input_attrs on such textarea controls.

Props melchoyce, westonruter, celloexpressions.
See #37439.
Fixes #38493.


git-svn-id: https://develop.svn.wordpress.org/trunk@38933 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-manager.php

@@ -3415,6 +3415,9 @@ final class WP_Customize_Manager {
 			'type'     => 'textarea',
 			'section'  => 'custom_css',
 			'settings' => array( 'default' => $custom_css_setting->id ),
+			'input_attrs' => array(
+				'placeholder' => __( "/*\nYou can add your own CSS here.\n\nClick the help icon above to learn more.\n*/" ),
+			)
 		) );
 	}