Feature suggestions for offline mode (kn service --target)

### Feature request

Some minor tuning for the `kn service create --target` offline mode:

* The success message should mention which file has been created. Until now it looks like the usual kn service create message without any context that it has operated in offline mode
* Empty sections should be filtered out in the file created. This might be a bit tricky, but I think it should be possible as a post-processing step.
* I got some feedback on the `--target` naming. Not everyone liked it and has a different association with it. Maybe we should revisit it. Let's discuss again on this ticket whether we want to change and maybe find a better alternative.

Start with the `kn service create --target` entry point and inspect merged pull request #1821 for the current offline-mode behavior. Check how the success message and generated file are handled, then determine whether empty sections and the `--target` name still need follow-up. Done means the requested behavior is agreed and covered by the relevant changes.

Written by the indexing model from the issue text.