Change --completion-script-bash option to completion bash by convention

[erhhung]

Proposed change

For new users especially, the shell auto-completion feature is invaluable. However, it was not obvious to discover whether this CLI tool even supported completions as it doesn't appear to be documented in its --help output. I had to search around to see the --completion-script-bash global option randomly mentioned in an issue discussion, and then determine by trial-and-error which additional shells were supported.

By popular convention, shell completion can be enabled by sourcing the output of the completion <shell> subcommand, and should be clearly documented in the help output, exactly as the sister command nsc has done.

Use case

Better discoverability and user experience.

Contribution

No response