Add snapshot command help documentation to RootCommand

[verschaevesiebe]

Description

Add inline help documentation for the experimental snapshot command in RootCommand.cs, including argument descriptions, all supported options (--mode, --tenant-id, --subscription-id, --resource-group, --location, --deployment-name), and usage examples. The help text was derived from the existing SnapshotArguments.cs and SnapshotCommand.cs implementations to ensure accuracy.

Example Usage

bicep snapshot params.bicepparam
bicep snapshot params.bicepparam --mode Overwrite
bicep snapshot params.bicepparam --mode Validate
bicep snapshot params.bicepparam --subscription-id 00000000-0000-0000-0000-000000000000 --resource-group my-rg

Checklist

• I have read and adhere to the [contribution guide](https://github.com/Azure/bicep/blob/main/CONTRIBUTING.md).

Microsoft Reviewers: Open in CodeFlow

[verschaevesiebe]

@microsoft-github-policy-service agree

[verschaevesiebe]

Feel free to keep this open until snapshot goes GA