Follow-up from "Proto/Client: update register key CLI with companion and consensus keys args"

The following discussion from !18317 (merged) should be addressed:

• @ACoquereau started a discussion: (+2 comments)

  Changes LGTM. I took a quick look at the documentation page https://tezos.gitlab.io/-/tezos/-/jobs/10342867279/artifacts/docs/_build/s023/cli-commands.html everything LGTM.

  Out of scope of this MR but it seems that we cannot output the options of the register command through --help or man

  octez-client register key toto as delegate --help 
  Error:
    Unterminated command, here are possible completions.
      register key <mgr> as delegate with consensus key <key> 
      register key <mgr> as delegate