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.


git-svn-id: https://develop.svn.wordpress.org/trunk@25860 602fd350-edb4-49c9-b593-d223f7449a82