Expand docs for return values of stats.binned_statistic

This is a proposal to improve the documentation for the return values
of `stats.binned_statistic` and its multi-dimensional variants.
I am not entirely happy with the proposed wording, but I think it is more
informative then what we have today and is a step in the right direction.

I have stuggeled for a while to understand what the indices mean and this is the best description I could come up with.