[Doc] Admin CLI does not document outputs

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### What issue do you find in Pulsar docs?

1. The current admin CLI docs does not provide documentation of the output format of the commands. Neither success nor failure, this makes it difficult to understand the output of some commands. I was specifically having issues with https://pulsar.apache.org/reference/#/3.0.x/pulsar-admin/broker-stats and the units of the various fields
2. After delving into the source it isn't obvious that if I wanted to go and add this documentation how to do this. The current docs are autogenerated from the jcommander annotations. JCommander doesn't provide a means to document the output of the commands since it is about building the program and documentation is secondary.

### What is your suggestion?

1. Create a pattern for adding this sort of output documentation.
2. Document new CLI commands

### Any reference?

_No response_

### Are you willing to submit a PR?

- [X] I'm willing to submit a PR!

Start with the generated Pulsar admin CLI reference, especially the broker-stats command, and trace how JCommander annotations produce its documentation. Define a consistent pattern for documenting success and failure output, including field units, then apply it to the relevant CLI commands and verify the generated reference.

Written by the indexing model from the issue text.