I've added "iterating..." comments.

I've chosen ```sizeof(entry)``` instead of ```sizeof(shell_command_t)``` because

1. it's shorter
2. I can't use typename in ```sizeof(command_lists)``` because that's an array and I don't want to use both styles in the same line.
3. For me "sizeof(lists) / sizeof(entry)" reads clearer than "sizeof(lists) / sizeof(<some_type_name>)"

convince me! ;)