Customer feedback | az communication email domain sender-username | UUF azDO #265489

### Describe the bug

This customer feedback is coming from UUF [265489](https://dev.azure.com/msft-skilling/Content/_workitems/edit/265489/).

### Engineering request

Please add parameter information for "required when". This is a constant misunderstanding across all of Azure CLI reference content and we are working to improve our parameter `short-summary` property. On a conditional parameter that ends up being listed under `### Optional`, inform the customer when `--username` is required.

#### Suggested parameter description
`A sender senderUsername to be used when sending emails.` becomes `A sender senderUsername to be used when sending emails. Required when .`

### Customer feedback

**Verbatim:**
--username is not optional, it is REQUIRED

Exception Details: (ObjectMissingRequiredProperty) Missing required property: username. Paths in payload: '$.properties.username' Code: ObjectMissingRequiredProperty Message: Missing required property: username. Paths in payload: '$.properties.username'

COME ON!! Just kidding, but seriously come on please. One ounce of effort towards accurate documentation please.

**Verbatim Id:**
0aea8f7b-bd2a-ef11-840a-000d3a5ca30a

**Live URL:**
https://learn.microsoft.com/en-us/cli/azure/communication/email/domain/sender-username

**GitHub URL:**
https://github.com/MicrosoftDocs/azure-docs-cli/blob/live/latest/docs-ref-autogen/communication/email/domain/sender-username.yml

### Related command

az communication email domain sender-username

### Errors

Exception Details: (ObjectMissingRequiredProperty) Missing required property: username. Paths in payload: '$.properties.username' Code: ObjectMissingRequiredProperty Message: Missing required property: username. Paths in payload: '$.properties.username'

### Issue script & Debug output

Exception Details: (ObjectMissingRequiredProperty) Missing required property: username. Paths in payload: '$.properties.username' Code: ObjectMissingRequiredProperty Message: Missing required property: username. Paths in payload: '$.properties.username'

### Expected behavior

n/a

### Environment Summary

Azure CLI 2.64.0

### Additional context

_No response_