Improve inline documentation for wp-includes/load.php.

Props ericlewis, DrewAPicture.
Fixes #24886.

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Mark the `MYSQL_NEW_LINK` constant as deprecated in 3.9+ as no equivalent to the `$new_link` parameter exists in `mysqli_*` functions.

Fixes #27933.

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Make sure to use 3-digit x.x.x style for two 3.9.0 `@since` versions.

Props netweb.
Fixes #28446.

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git-svn-id: http://core.svn.wordpress.org/trunk@28476 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix SQL generation when `meta_query` has an `'relation' => 'OR'` for its queries and wants to `'orderby' => 'meta_value'`.

Adds unit test.

Props jackreichert.
Fixes #25538.


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git-svn-id: http://core.svn.wordpress.org/trunk@28477 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Improve inline documentation for the `__construct()` and `add()` methods in `WP_Error` class.

Props jdgrimes.
Fixes #28203.

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