ssh: Fix dialyzer type specs and documentation

Similar to d9ebfb8. The wrong specs were leading to dialyzer warnings
like this in our application since R16B03:

    The pattern 'ok' can never match the type {'error',_}.
    The pattern {'error', {'already_started', 'ssh'}} can never match the type 'ok'.
    The pattern {'error', _} can never match the type {'open_error',_,string(),string()}.