Clarify input vs. output directories in the validator CLI arguments

This might be for a separate discussion/PR, but I find the argument names a little misleading: it's not immediately clear whether these are input directories or output directories, for example:
- `genesis_block_directory` is an output location, but
- `genesis_config_file` is an input location (here, path, not a directory, but the same argument applies)

_Originally posted by @mmagician in https://github.com/0xMiden/node/pull/1774#discussion_r2952439567_