Properly account for text overflow in `.wp-playlist-caption` by adding `display: block`.

Props celloexpressions.
See #27923.


Built from https://develop.svn.wordpress.org/trunk@28172


git-svn-id: http://core.svn.wordpress.org/trunk@28003 1a063a9b-81f0-0310-95a4-ce76da25c4cd