Add note about why double-serialization is done. props SergeyBiryukov. fixes #16597

git-svn-id: https://develop.svn.wordpress.org/trunk@18807 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/functions.php

@@ -1037,6 +1037,8 @@ function maybe_serialize( $data ) {
 	if ( is_array( $data ) || is_object( $data ) )
 		return serialize( $data );
 
+	// Double serialization is required for backward compatibility.
+	// See http://core.trac.wordpress.org/ticket/12930
 	if ( is_serialized( $data ) )
 		return serialize( $data );