Document MPI communicator setup protocol #1747

Job	Run time
test_bmi_cpp (ubuntu-latest)	2m 39s
test_bmi_fortran (ubuntu-latest)	2m 46s
test_bmi_cpp (macos-latest)	3m 49s
test_mpi_remote_nexus (ubuntu-latest)	1m 21s
test_bmi_fortran (macos-latest)	9m 2s
test_pet (ubuntu-latest)	3m 13s
test_unit (ubuntu-latest)	4m 59s
test_pet (macos-latest)	4m 42s
test_unit (macos-latest)	5m 28s
test_bmi_c (ubuntu-latest)	3m 32s
test_bmi_multi (ubuntu-latest)	3m 49s
test_bmi_multi (macos-latest)	12m 16s
test_bmi_c (macos-latest)	4m 53s
test_bmi_python (ubuntu-latest)	3m 34s
test_bmi_python (macos-latest)	5m 9s
	1h 11m 12s

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