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.

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Move upgrader_process_complete for core to its proper place in Core_Upgrader.

This means it will be firing as a JS redirect is taking place if the update is from pre-3.4. Acceptable.

props dd32.
fixes #25659.

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Improve inline documentation for the `wp_get_sites()` return value.

Fixes #25645.

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Updates: Saner stats and rollback handling. WP.org now collects basic stats on non-auto updates as they have been hugely informative.

props dd32 for initial patch.
fixes #25657.

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Twenty Fourteen: further revise primary navigation functionality, simplify mobile navigation, and clean up the main JS file. Props obenland, iamtakashi. See #25554.
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Twenty Fourteen: use SVG images instead of CSS3 gradients for featured image background pattern. Props iamtakashi.

 * Fixes laggy loading in Firefox, see #25600.
 * Significantly reduces CPU usage.
 * Scalable and crisp on HiDPI/retina screens.
 * SVG is support in IE9 which doesn't support CSS gradients.
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Updates: Disable maintenance mode once we've finished copying files, to minimize disruption.

props dd32.
fixes #25655.

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Always apply the posts_search filter. Restores 3.6 behavior.

props SergeyBiryukov.
fixes #25664.

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Spell out duplicate hook locations.

props DrewAPicture.
fixes #25658.

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Only enforce disk free space checks when doing background updates.

see #25652.

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New, better error message when we realize we won't be able to copy a file and continue with the update.

fixes #25652.

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"some files" instead of "certain files". Less stuffy.

props markjaquith.
see #25652.

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Show the nag we show for a stuck .maintenance file when an auto update failed.

see #25654.

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Make some changes to the email templates to account for the fact that an auto update offer might not be to the latest version.

For example, 3.7.2 install is served a 3.7.3 update, but the current version is 3.8.1.

This commit also allows for core update notification emails to be sent even when we are unable to run an auto update due to filesystem configuration (VCS checkout or file permissions). But, since these emails go through WP_Automatic_Updater, they will not be sent if the updater is outright disabled.

fixes #25654.

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