Add missing phpdoc block for `export_date_options()`.

Props siobhan for the initial patch. See #20425

Built from https://develop.svn.wordpress.org/trunk@25490


git-svn-id: http://core.svn.wordpress.org/trunk@25411 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add phpdoc blocks in wp-admin/credits.php.

Props siobhan, markoheijnen.
Fixes #20425.

Built from https://develop.svn.wordpress.org/trunk@25491


git-svn-id: http://core.svn.wordpress.org/trunk@25412 1a063a9b-81f0-0310-95a4-ce76da25c4cd