Add docs to the woefully under-documented `get_pages()`.

Props DrewAPicture.
Fixes #25186.


Built from https://develop.svn.wordpress.org/trunk@25266


git-svn-id: http://core.svn.wordpress.org/trunk@25232 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fourteen: further styling for medialement players, props iamtakashi. Fixes #25209.
Built from https://develop.svn.wordpress.org/trunk@25267


git-svn-id: http://core.svn.wordpress.org/trunk@25233 1a063a9b-81f0-0310-95a4-ce76da25c4cd