Inline documentation for the `WP_Date_Query` class in wp-includes/date.php.

- Adds a complete hash notation for the `WP_Date_Query` arguments array.
- Adds missing documentation for the `date_query_valid_columns` and `get_date_sql` filter hooks.

Props aeg0125 for the incremental patches.
Fixes #25552.

Built from https://develop.svn.wordpress.org/trunk@25860


git-svn-id: http://core.svn.wordpress.org/trunk@25772 1a063a9b-81f0-0310-95a4-ce76da25c4cd