Docs: Add documentation for the $type, $mime_type, and $button_labels properties in WP_Customize_Upload_Control.

Follow-up to [20319], [30309], [31698], [32023].

Props subrataemfluence.
See #44411.

git-svn-id: https://develop.svn.wordpress.org/trunk@49679 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/customize/class-wp-customize-upload-control.php

@@ -15,10 +15,31 @@
  * @see WP_Customize_Media_Control
  */
 class WP_Customize_Upload_Control extends WP_Customize_Media_Control {
-	public $type          = 'upload';
-	public $mime_type     = '';
+	/**
+	 * Control type.
+	 *
+	 * @since 3.4.0
+	 * @var string
+	 */
+	public $type = 'upload';
+
+	/**
+	 * Media control mime type.
+	 *
+	 * @since 4.1.0
+	 * @var string
+	 */
+	public $mime_type = '';
+
+	/**
+	 * Button labels.
+	 *
+	 * @since 4.1.0
+	 * @var array
+	 */
 	public $button_labels = array();
-	public $removed       = '';   // Unused.
+
+	public $removed = '';         // Unused.
 	public $context;              // Unused.
 	public $extensions = array(); // Unused.