Update MPI Best Practices docs

[monaghaa]

Not all of the above are clearly stated in our MPI best practices does for Alpine

https://curc.readthedocs.io/en/latest/programming/MPIBestpractices.html#running-mpi-on-alpine

The basics are:

• use the --constraint=ib flag when running MPI jobs on Alpine (this is already stated)
• Always specify the number of nodes explicitly to ensure optimal message passing performance, and that loads are balanced properly across nodes for multi-node job: If using less than 64 cores, use --nodes=1. If > 64 cores, increment in units of nodes/cores (--ntasks=128 & --nodes=2; or --ntasks=2 & --nodes=4)
• Similar concept applies to Blanca nodes, though core counts differ.