Customer feedback | az stack sub validate | missing information | validate what, how, why?

### Describe the bug

This GitHub issue is in response to MicrosoftDocs/azure-docs-cli customer feedback [4639](https://github.com/MicrosoftDocs/azure-docs-cli/issues/4639).

### Engineering request

Please add more information to the reference command description. What does this command validate? Why does a customer want to do this? Use the `long-summary` property if needed.

There is one conceptual article that uses the commands in this subgroup, [Deployment stacks](https://learn.microsoft.com/en-us/azure/azure-resource-manager/bicep/deployment-stacks?tabs=azure-cli), but it doesn't address validation.

### Customer feedback
**Link:**
https://learn.microsoft.com/en-us/cli/azure/stack/sub?view=azure-cli-latest#az-stack-sub-validate

**Verbatim:**
Here it says "Validate a deployment stack at subscription scope.", which sounds nice, but what are you validating? I don't understand what this command is doing at all.

If I change something in my bicep file or parameter file it just says "looks cool 👍" with a list of "validatedResources" (including resources that don't exist yet or that I modified)

I had hoped it would give me a list of what would change (but sadly, this doesn't seem to exist at all for deployment stacks?)...

**Page URL**
https://learn.microsoft.com/en-us/cli/azure/stack/sub?view=azure-cli-latest

**Content source URL**
https://github.com/MicrosoftDocs/azure-docs-cli/blob/main/latest/docs-ref-autogen/stack/sub.yml

### Related command

az stack sub validate

### Errors

n/a

### Issue script & Debug output

n/a

### Expected behavior

n/a

### Environment Summary

Azure CLI 2.64.0

### Additional context

_No response_