Humanize CLI command group descriptions a bit

[booxter]

What does this PR do?

Descriptions were repetitive and verbose, some were truncated by click
rendering (trailing ... ellipsis). This is an attempt to simplify /
shorten / clarify the descriptions.

Test Plan

Before:

 llama-stack-client --help
Usage: llama-stack-client [OPTIONS] COMMAND [ARGS]...

  Welcome to the LlamaStackClient CLI

Options:
  --version        Show the version and exit.
  --endpoint TEXT  Llama Stack distribution endpoint
  --api-key TEXT   Llama Stack distribution API key
  --config TEXT    Path to config file
  --help           Show this message and exit.

Commands:
  configure          Configure Llama Stack Client CLI
  datasets           Query details about available datasets on Llama...
  eval               Run evaluation tasks
  eval_tasks         Query details about available eval tasks type on...
  inference          Query details about available inference endpoints on...
  inspect            Query details about available versions on Llama...
  models             Query details about available models on Llama Stack...
  post_training      Query details about available post_training...
  providers          Query details about available providers on Llama...
  scoring_functions  Manage scoring functions
  shields            Query details about available safety shields on...
  toolgroups         Query details about available toolgroups on Llama...
  vector_dbs         Query details about available vector dbs on...

After:

 llama-stack-client --help
Usage: llama-stack-client [OPTIONS] COMMAND [ARGS]...

  Welcome to the LlamaStackClient CLI

Options:
  --version        Show the version and exit.
  --endpoint TEXT  Llama Stack distribution endpoint
  --api-key TEXT   Llama Stack distribution API key
  --config TEXT    Path to config file
  --help           Show this message and exit.

Commands:
  configure          Configure Llama Stack Client CLI.
  datasets           Manage datasets.
  eval               Run evaluation tasks.
  eval_tasks         Manage evaluation tasks.
  inference          Inference (chat).
  inspect            Inspect server configuration.
  models             Manage GenAI models.
  post_training      Post-training.
  providers          Manage API providers.
  scoring_functions  Manage scoring functions.
  shields            Manage safety shield services.
  toolgroups         Manage available tool groups.
  vector_dbs         Manage vector databases.