Document DOMDocument::schemaValidate/Source() flags parameter.

See https://github.com/php/php-src/pull/325

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Provided by Chris Wright (phpdoc@daverandom.com)

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@331156 c90b9560-bf6c-de11-be94-00142212c4b1