Adding DOC explication for the method FIRST/LAST of a Select

[DioxideHydrogen]

I recently have a issue with the first/last method of select, and i think that if these method was more explained i dont have problem and it will be clearer for those who read the documentation.

I needed go to code of Select.php to understand that the method first order the select ascending, and the last descending.

I believe that if the query already have a order, that methods don't need order again.