nproc: move help strings to markdown file

[piotrkwiecinski]

#4368

nproc --help outputs the following:

target/debug/nproc --help
Print the number of cores available to the current process.
If the `OMP_NUM_THREADS` or `OMP_THREAD_LIMIT` environment variables are set, then
they will determine the minimum and maximum returned value respectively.

Usage: ./target/debug/nproc [OPTIONS]...

Options:
      --all         print the number of cores available to the system
      --ignore <N>  ignore up to N cores
  -h, --help        Print help
  -V, --version     Print version

[sylvestre]

thanks for adding the ` ;)

[cakebaker]

Thanks!