Based on svn repository revision 6855.

wp-admin/index.php:73
$sentence = sprintf( __( 'You have %1$s, contained within %2$s and %3$s.' ), $post_type_text, $cats_text, $tags_text );

$post_type_text can be singular/plural, therefore the whole sentence can be singular/plural, too.
$sentence = sprintf( __ngettext( 'You have %1$s, contained within %2$s and %3$s.', 'You have %1$s, contained within %2$s and %3$s.', $post_type_text, $cats_text, $tags_text) );

In my language the word form "contained" in this sentence changes depending upon whether %1$s is singular or plural.