Use `selected()` where appropriate in `touch_time()`, `page_template_dropdown()`, and `parent_dropdown()`. Also, add proper docs.

Props meloniq, DrewAPicture.
Fixes #25889.



git-svn-id: https://develop.svn.wordpress.org/trunk@27188 602fd350-edb4-49c9-b593-d223f7449a82