Add a missing return description for `has_header_image()`.

See [31224]. See #31888.


git-svn-id: https://develop.svn.wordpress.org/trunk@32048 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -1039,7 +1039,9 @@ function display_header_text() {
  *
  * @since 4.2.0
  *
- * @return bool
+ * @see get_header_image()
+ *
+ * @return bool Whether a header image is set or not.
  */
 function has_header_image() {
 	return (bool) get_header_image();