[doc] Clarify the difference between `-v` and `--verbosity` in help messages
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Issue body
### What needs to get done
From the output of `snapcraft <command> --help`, I can't tell the difference between:
```
-v, --verbose: Show debug information and be
more verbose
```
and
```
--verbosity: Set the verbosity level to
'quiet', 'brief', 'verbose',
'debug' or 'trace'
```
I'm guessing:
- `-verbose` prints debug logs to the local stdout
- `--verbosity` prints various levels of logs to the remote build log file
It would be handy if the use cases were clearly differentiated in the help message.
### Why it needs to get done
Avoid unnecessary confusion.
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