[Feature request] Show command group help when subcommand is missing

[jiasli]

For example, if no subcommand is provided to docker image, the help message is displayed:

> docker image

Usage:  docker image COMMAND

Manage images

Commands:
  build       Build an image from a Dockerfile
  history     Show the history of an image
  import      Import the contents from a tarball to create a filesystem image
  inspect     Display detailed information on one or more images
  load        Load an image from a tar archive or STDIN
  ls          List images
  prune       Remove unused images
  pull        Pull an image or a repository from a registry
  push        Push an image or a repository to a registry
  rm          Remove one or more images
  save        Save one or more images to a tar archive (streamed to STDOUT by default)
  tag         Create a tag TARGET_IMAGE that refers to SOURCE_IMAGE

Run 'docker image COMMAND --help' for more information on a command.

Nowadays, Azure CLI shows an error message:

$ az group
the following arguments are required: _subcommand

TRY THIS:
az group create --location westus --resource-group MyResourceGroup
Create a new resource group in the West US region.

az group delete --resource-group MyResourceGroup
Delete a resource group.

az group list --query "[?location=='westus']"
List all resource groups located in the West US region.

https://docs.microsoft.com/en-US/cli/azure/group#az_group_list
Read more about the command in reference docs

It will be more user-friendly to just show the help message of az group --help directly.

[yonzhan]

help refinement

[jiasli]

A similar error message is shown if no top-level command group is given:

> az --test
the following arguments are required: _command_package