Feature request: Improve documentation around output format

[avi-cenna]

It would be nice to improve the documentation, either in a markdown document or just the command line help string, for the output flag. Right now it's not clear that the JSON output format is actually different from the default one.

Here is an excerpt of the help string. As we can see, the --output flag doesn't have a very clear explanation. I think I'd be happy to tackle this and submit a PR. But I'd like some confirmation from the maintainer that this type of a change would be acceptable.

 ❯ kaf help consume
Consume messages

Usage:
  kaf consume TOPIC [flags]

Flags:
  -g, --group string            Consumer Group to use for consume
  -h, --help                    help for consume
      --key-proto-type string   Fully qualified name of the proto key type. Example: com.test.SampleMessage
  -l, --limit-messages int      Limit messages per partition
      --offset string           Offset to start consuming. Possible values: oldest, newest, or integer. (default "oldest")
      --output OutputFormat     Set output format messages: default, raw (without key or prettified JSON), json (default default)
  -n, --tail int32              Print last n messages per partition