Next steps on listrev documentation

[jcfreeman2]

Right now the documentation (https://dune-daq-sw.readthedocs.io/en/latest/packages/listrev/) does what it says it's going to do, in that it shows you how to create a vector of ints, reverse it, and confirm that it was reversed. Since my understanding of the purpose of listrev is as an example of DAQModule behavior, it might be good for its documentation to go a little more in depth on how these DAQModules work and how they're representative of DAQModules in general. Of course, we'd want to avoid repeating any material on DAQModules that might get added to the appfwk documentation.