[DOC] Relocate note in main CLI help about existence of subcommand help to more prominent location

[bskinn]

          Aha, I need to add to the base help message (`sphobjinv -h`) to indicate that there's additional help for each subcommand (`sphobjinv convert -h` and `sphobjinv suggest -h`).

Originally posted by @bskinn in #279 (comment)