Documentation for Array#sample

Hi Thiago, we normally do not document backports because they are known methods, or methods with existing documentation anyway. We do document small differences if there are any, because a backport is assumed otherwise to behave exactly the same in all edge cases, invalid arguments, etc.

That being said that's not a hard rule and the patch is welcome, could you please commit it yourself via docrails? I'll merge it into master later. docrails has public write access, just push the changes.

Thanks!

Assuming this commit in docrails to resolve this ticket http://github.com/lifo/docrails/commit/0175c470c01c22db2f54c3912958...