Document which versions of Intel MPI and/or MPICH can achieve direct GPU-GPU communications

Summary

Users and sysadmins need to know what to expect will work and should be able to read it in the install guide.

Use cases

Users will see better performance if their GROMACS build uses an MPI library that does such communication.

Impact

Users on PVC in 2023.

Detailed description

Intel MPI won't be available until November or so, so aspects of this documentation fix will go in quite late.