possible documentation update for the atomicAdd

[YonsiG]

Can we have a documentation of the atomic functions in alpaka? I found it confusing to use, and when I check the documentation on GitHub, I found it empty.

https://alpaka-group.github.io/alpaka/Positioning_8hpp_source.html#l00012

alpaka::hierarchy::Threads{}: atomic across all threads in a block;
alpaka::hierarchy::Blocks{}: atomic across all blocks in a grid;
alpaka::hierarchy::Grids{}: atomic for all grids (i.e. kernels) on the device.

Thanks

[SimeonEhrig]

@YonsiG We discussed you issue and we will provide some documentation. Unfortunately, this will not happen before next week or maybe the week after. The developer who has the deep knowledge to write the documentation is pretty busy at the moment.