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-control.php

@@ -532,7 +532,7 @@ class WP_Customize_Control {
 					if ( ! empty( $this->description ) ) : ?>
 						<span class="description customize-control-description"><?php echo $this->description; ?></span>
 					<?php endif; ?>
-					<textarea rows="5" <?php $this->link(); ?>><?php echo esc_textarea( $this->value() ); ?></textarea>
+					<textarea rows="5" <?php $this->input_attrs(); ?> <?php $this->link(); ?>><?php echo esc_textarea( $this->value() ); ?></textarea>
 				</label>
 				<?php
 				break;

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*/" ),
+			)
 		) );
 	}