message: Add sphinx documentation describing attempts to read using IOManager on
  increased or decreased numbers of processors.
diff: @@ -53,6 +53,17 @@ to a set of files and read a Group from existing files.
  IOManager optionally uses the SCR library for scalable I/O
  management (such as using burst buffers if available).
  +In typical usage, a run that calls ``read()`` on a certain set of files
  +should be executed on the same number of MPI ranks as the run that created
  +those files with a ``write()`` call. Still a ``read()`` call, if using
  +the "sidre_hdf5" protocol, can work when called from a greater number of
  +processors. If ``write()`` was executed on N ranks and ``read()`` is called
  +while running on M ranks (M > N), then data will be read into ranks 0 to N-1,
  +and all ranks higher than N-1 will receive no data.
  +
  +If ``read()`` is called in an attempt to read data that was created on a
  +larger number of processors than the current run, an error will occur.
  +
  In the following example, an IOManager is created and used to write the contents
  of the Group "root" in parallel.
  