Document OpenMPI-ROMIO/HDF5/Chunking issue

[franzpoeschel]

open-mpi/ompi#7795

Ideally, we would deactivate chunking by default when detecting use of OpenMPI 4.* + ROMIO, but the only way I found to figure this out is by environment variable:

$ mpirun --mca io ompio -n 1 env | grep 'OMPI_MCA_io'
OMPI_MCA_io=ompio
$ mpirun --mca io romio -n 1 env | grep 'OMPI_MCA_io'
OMPI_MCA_io=romio

Imo, trying to parse this env variable is not worth the effort for a bug that only occurs when not using defaults.

X-ref: open-mpi/ompi#11715