cmd/completion: update usage string with tips

General MR

Summary

Update doc strings with two more useful tips for configuring completions in environments.

Bash: requires the bash-completion supporting package, otherwise you get:

 bash: _get_comp_words_by_ref: command not found

ZSH: The completions directory may not exist and you'll receive:

edf00f38d1ae# bbctl completion zsh > "${fpath[1]}/_bbctl"
zsh: no such file or directory: /usr/local/share/zsh/site-functions/_bbctl

Linked Issue

Part of #51 (closed)

Upgrade Notices

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