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Unfortunately, I am not aware of any stand alone documentation of the XoopsDatabase classes.
Good news, the classes do have some useful internal documentation:
http://api.xoops.org/2.5.9/class-XoopsMySQLDatabase.html
In general, the methods correlate closely to the mysqli functions of PHP, so the documentation there is useful:
http://php.net/manual/en/book.mysqli.php