Rewrite PDOStatement::fetch() docs

[Girgias]

Remove personalization Add constant tags Reorder fetch modes in a more logical way Update an example and make it runnable Remove useless wrapping <para> Use <simpara> when possible

Now I do wonder if it makes sense to detail this all here or if we should refer back to https://www.php.net/manual/en/pdo.constants.fetch-modes.php instead?