Add a note on launchers from active lsst PR lsst/ctrl_bps_parsl#36 wh…

…ich made me want to clarify this

blocks.rst

@@ -79,6 +79,8 @@ To support this, some providers take a ``launcher`` parameter, which understands
 
 All of the included launchers live in `parsl.launchers.launchers <https://github.com/Parsl/parsl/blob/3f2bf1865eea16cc44d6b7f8938a1ae1781c61fd/parsl/launchers/launchers.py>`_ and usually consist of shell scripting around something like ``mpiexec`` or ``srun``.
 
+.. todo:: a paragraph that in traditional HPC workloads, this launcher command is often responsible for starting multiple copies of your code on the same node - so if you wanted 24 cores used for an MPI code, you might use mpirun (TODO: processes_per_node param) to start 24 copies which would run in parallel. This is not how things work with parsl block workers: both the process worker pool and the WQ/TV equivalents usually manage all the tasks on a node from a single worker. So if you're feeling the temptation to make your launcher launch multiple copies of the pilot job worker, maybe there's something else going wrong?
+
 .. index:: scaling
 
 Choosing when to start or end a block