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

Props meloniq, DrewAPicture.
Fixes #25889.


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Remove code from a previous patch. See [27188].


Built from https://develop.svn.wordpress.org/trunk@27189


git-svn-id: http://core.svn.wordpress.org/trunk@27048 1a063a9b-81f0-0310-95a4-ce76da25c4cd