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

Props ericlewis, DrewAPicture.
Fixes #24886.

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git-svn-id: http://core.svn.wordpress.org/trunk@28474 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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.

Built from https://develop.svn.wordpress.org/trunk@28657


git-svn-id: http://core.svn.wordpress.org/trunk@28475 1a063a9b-81f0-0310-95a4-ce76da25c4cd