Add a 6th (!) attribute to `wp_get_attachment_link()` to allow `aria-describedby` to be added to gallery output.

Props joedolson, DrewAPicture, rianrietveld.
Fixes #27402.


git-svn-id: https://develop.svn.wordpress.org/trunk@29914 602fd350-edb4-49c9-b593-d223f7449a82