decK documentation does not details return values

### Where is the problem?

https://docs.konghq.com/deck/1.8.x/reference/deck/

### What happened?

"We needed the return values for a deck sync/diff/dump calls so that we can use that in our pipeline script. The documentation does not provide them"

### What did you expect to happen?

"We should have a paragraph per each subcommand specifying the return values"

### Code of Conduct and Community Expectations

- [X] I agree to follow this project's Code of Conduct
- [X] I agree to abide by the Community Expectations

Open the linked decK reference page and review the sync, diff, and dump subcommand sections. Confirm the return values for each subcommand, then document them in a separate paragraph per subcommand; the page should clearly describe these values when complete.

Written by the indexing model from the issue text.