[Documentation] Behavior of block scans with initial value param

[pgera]

Hi, I spent quite some time chasing down a bug which ended up being a misunderstanding of the behavior of block scans when an initial val param is given. It turns out that the block aggregate does not include the initial val. i.e., The exclusive sum of {1,2,3} with initial val 10 is {10, 11, 13 }, but the block aggregate is not 16. It is 6. This was not clear from the documentation and the general expectation of an aggregate.