Change descriptions for --imgpkg-lock-output and --lock-output

For users unfamiliar with kbld/imgpkg, it may be difficult to distinguish between the following flag options:

```
--imgpkg-lock-output string File path to emit images lockfile with resolved image references
--lock-output string File path to emit configuration with resolved image references
```

There should be more added to the descriptions about when to use each. Something detailing that `--lock-output` does not produce an `images.yml` for imgpkg might help users looking for more information through `kbld --help`.

Start by running `kbld --help` and locating the descriptions for `--imgpkg-lock-output` and `--lock-output`. Update the help text so users can distinguish their purposes, including that `--lock-output` does not produce an `images.yml` for imgpkg; done when both options are clearly differentiated.

Written by the indexing model from the issue text.