Twenty Nineteen: Document the twentynineteen_can_show_post_thumbnail filter.

Follow-up to [43808].

Props pmbaldha, sabernhardt, kishanjasani, mukesh27.
Fixes #63645.

git-svn-id: https://develop.svn.wordpress.org/trunk@61062 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-content/themes/twentynineteen/inc/helper-functions.php

@@ -11,7 +11,16 @@
  * Determines if post thumbnail can be displayed.
  */
 function twentynineteen_can_show_post_thumbnail() {
-	return apply_filters( 'twentynineteen_can_show_post_thumbnail', ! post_password_required() && ! is_attachment() && has_post_thumbnail() );
+	$show_post_thumbnail = ! post_password_required() && ! is_attachment() && has_post_thumbnail();
+
+	/**
+	 * Filters whether to show post thumbnail.
+	 *
+	 * @since Twenty Nineteen 1.0
+	 *
+	 * @param bool $show_post_thumbnail Whether to show post thumbnail.
+	 */
+	return apply_filters( 'twentynineteen_can_show_post_thumbnail', $show_post_thumbnail );
 }
 
 /**