doc: MPIEXEC_PREFIX_DEFAULT doesn't work

[simonbyrne]

I tried using the MPIEXEC_PREFIX_DEFAULT environment variable (which is suggested on the man page) but it doesn't seem to have an effect:

> MPIEXEC_PREFIX_DEFAULT=1 mpiexec -n 2 echo "hello world"
hello world
hello world

However the -prepend-rank option (which isn't mentioned on the man page, but is mentioned under mpiexec --help) does work correctly:

> mpiexec -prepend-rank -n 2 echo "hello world"
[0] hello world
[1] hello world

(Discovered in JuliaParallel/MPI.jl#360)

[hzhou]

Thanks for reporting.

[tkoehring]

I have submitted a PR that fixes this issue. There was no code that handle this environment variable and we felt that it should stay so I've added some.