Use proper help format for "completion"

[marckhouzam]

What this PR does / why we need it

The completion and generate-all-docs commands' help were using the root command's help format by mistake. This PR fixes that.

Also, this PR improves the output of the list of shells in the help output of the completion command.

Which issue(s) this PR fixes

Fixes # N/A

Describe testing done for PR

Before this commit, notice below

1. the invalid use of [command]
2. the invalid use of Available command groups
3. the poor formatting of the list [bash zsh fish powershell]

$ ~/bin/tanzu.v1.0.0 completion -h

Output shell completion code for the specified shell [bash zsh fish powershell].

The shell completion code must be evaluated to provide completion. See Examples
for how to perform this for your given shell.

Note for bash users: make sure the bash-completions package has been installed.

Usage:
  tanzu completion [command]

Examples:

# Bash instructions:

  ## Load only for current session:
  source <(tanzu completion bash)

  ## Load for all new sessions:
  tanzu completion bash >  $HOME/.config/tanzu/completion.bash.inc
  printf "\n# Tanzu shell completion\nsource '$HOME/.config/tanzu/completion.bash.inc'\n" >> $HOME/.bash_profile

  ## NOTE: the bash-completion package must be installed.

# Zsh instructions:

  ## Load only for current session:
  autoload -U compinit; compinit
  source <(tanzu completion zsh)
  compdef _tanzu tanzu

  ## Load for all new sessions:
  echo "autoload -U compinit; compinit" >> ~/.zshrc
  tanzu completion zsh > "${fpath[1]}/_tanzu"

# Fish instructions:

  ## Load only for current session:
  tanzu completion fish | source

  ## Load for all new sessions:
  tanzu completion fish > ~/.config/fish/completions/tanzu.fish

# Powershell instructions:

  ## Load only for current session:
  tanzu completion powershell | Out-String | Invoke-Expression

  ## Load for all new sessions:
  Add the output of the above command to your powershell profile.

Available command groups:


Flags:
  -h, --help   help for completion

Use "tanzu completion [command] --help" for more information about a command.
$ ~/bin/tanzu.v1.0.0 generate-all-docs -h
Generate Cobra CLI docs for all plugins installed

Usage:
  tanzu generate-all-docs [command]

Available command groups:


Flags:
  -d, --docs-dir string   destination for docs output (default "docs/cli/commands")
  -h, --help              help for generate-all-docs

Use "tanzu generate-all-docs [command] --help" for more information about a command.

After this commit, notice the help format

1. does not use [command] anymore
2. no longer uses Available command groups
3. the better formatting of the lists [bash zsh fish powershell]

$ tz completion -h
Output shell completion code for the specified shell (bash, zsh, fish, powershell).

The shell completion code must be evaluated to provide completion. See Examples
for how to perform this for your given shell.

Note for bash users: make sure the bash-completions package has been installed.

Usage:
tanzu completion [bash|zsh|fish|powershell]

Examples:

# Bash instructions:

  ## Load only for current session:
  source <(tanzu completion bash)

  ## Load for all new sessions:
  tanzu completion bash > $HOME/.config/tanzu/completion.bash.inc
  printf "\n# Tanzu shell completion\nsource '$HOME/.config/tanzu/completion.bash.inc'\n" >> $HOME/.bashrc

  ## NOTE: the bash-completion OS package must also be installed.

  ## If you invoke the 'tanzu' command using a different name or an alias such as,
  ## for example, 'tz', you must also include the following in your $HOME/.bashrc
  complete -o default -F __start_tanzu tz

# Zsh instructions:

  ## Load only for current session:
  autoload -U compinit; compinit
  source <(tanzu completion zsh)

  ## Load for all new sessions:
  echo "autoload -U compinit; compinit" >> $HOME/.zshrc
  tanzu completion zsh > "${fpath[1]}/_tanzu"

  ## Aliases are handled automatically, but if you have renamed the actual 'tanzu' binary to,
  ## for example, 'tz', you must also include the following in your $HOME/.zshrc
  compdef _tanzu tz

# Fish instructions:

  ## Load only for current session:
  tanzu completion fish | source

  ## Load for all new sessions:
  tanzu completion fish > $HOME/.config/fish/completions/tanzu.fish

  ## Aliases are handled automatically, but if you have renamed the actual 'tanzu' binary to,
  ## for example, 'tz', you must also include the following in your $HOME/.config/fish/config.fish
  complete --command tz --wraps tanzu

# Powershell instructions:

  ## Load only for current session:
  tanzu completion powershell | Out-String | Invoke-Expression

  ## Load for all new sessions:
  printf "\n# Tanzu shell completion\ntanzu completion powershell | Out-String | Invoke-Expression" >> $PROFILE

  ## If you invoke the 'tanzu' command using a different name or an alias such as,
  ## for example, 'tz', you must also include the following in your powershell $PROFILE.
  Register-ArgumentCompleter -CommandName 'tz' -ScriptBlock ${__tanzuCompleterBlock}

Flags:
  -h, --help   help for completion

$ tz completion
[x] : shell not specified, choose one of: bash, zsh, fish, powershell

$ tz generate-all-docs -h
Generate Cobra CLI docs for all plugins installed

Usage:
tanzu generate-all-docs [flags]

Flags:
  -d, --docs-dir string   destination for docs output (default "docs/cli/commands")
  -h, --help              help for generate-all-docs

Release note

Use proper help format for "tanzu completion"

Additional information

Special notes for your reviewer

[vuil]

Like it! Thanks for the cleanup. lgtm