Document CLI UI guidelines

@aaronshurley commented on [Tue Mar 02 2021](https://github.com/vmware-tanzu/carvel-community/issues/19)

Flag guidelines:
- `--dangerous-*`
- `--*-alpha`

Examples of current inconsistencies:

- running `kapp` produces help msg and error
- vendir and imgpkg on the other hand just produce error message with list of sub commands
- ytt and kbld return successfully (since they have root commands registered)

_Note: This issue originated from https://github.com/k14s/design-docs/issues/2 but has been moved here as a part of the carvel rename._

Compare the documented command-line behavior of kapp, vendir, imgpkg, ytt, and kbld, starting with their help and error output. Define consistent guidelines for flags such as --dangerous-* and --*-alpha and for root-command behavior, then document the agreed conventions.

Written by the indexing model from the issue text.