Add some clarification/context to MPI code

[ocaisa]

We had a workshop yesterday and there were some questions about why things are done in a particular way for the MPI example

• Why the use of comm.Barrier()
• Why do count_item = comm.scatter(counts, root=0) when you know there is no useful information to scatter?

These kinds of lines in the example are done quite deliberately and point to some of the intricacies of MPI, we should probably make more effort to explain why we need to use them.

[ocaisa]

The example in general has massive overlap with https://carpentries-incubator.github.io/hpc-intro/16-parallel/index.html so we also need to figure out how to maintain the two without double the effort.