log-format or log_format?

Docs use `log-format` but output is `log_format`.  Which should be used?  Is this valid output?  Here is my current output which isn't working:

```
time-format %T
date-format %d/%b/%Y
log_format nginx
log_format COMBINED
exclude-ip 192.168.1.1-192.168.1.255
```

Now I cannot find `log_format nginx` anywhere in the documents.  