No clear documentation about the log level integer and human-readable description

microceph 19.2.0+snap9ecc402e1e 1280 squid/edge canonical✓ held

https://canonical-microceph.readthedocs-hosted.com/en/latest/how-to/change-log-level/

```
# microceph log --help
Manage microceph logs

Usage:
microceph log [flags]
microceph log [command]

Available Commands:
get-level Get the current log level, as an integer
set-level Set the log level for the microceph daemon

Global Flags:
-d, --debug Show all debug messages
-h, --help Print help
--state-dir Path to store state information
-v, --verbose Show all information messages
--version Print version number

Use "microceph log [command] --help" for more information about a command.
```

There is no information about:
- what kind of log levels are available (error, warning, info, debug, etc.)
- what's the mapping between the integer and the human readable log level (looks like 2=error, 3=warning, 4=info, 5=debug)

For example, the initial setting is `0`.

```
# microceph log get-level
0
```

But there is no explanation about what is `0`.

Start with the linked change-log documentation page and the `microceph log get-level` and `set-level` command help. Verify the available human-readable levels and their integer mapping, including the meaning of the initial value `0`, then update the page so users can interpret and configure log levels.

Written by the indexing model from the issue text.