[Doc]: Get-AzCosmosDBMongoDBCollectionThroughput returns 404 if run on a shared throughput collection

### Description

As titled, if the command is run on a shared throughput collection, we would only get the generic 404 error. The behavior is the same for both PS and bash CLI.

```text
Get-AzCosmosDBMongoDBCollectionThroughput: Entity with the specified id does not exist in the system. More info: https://aka.ms/cosmosdb-tsg-not-found
```

The ask here is to improve the self-help error message to make it stand out more.

Start by reproducing Get-AzCosmosDBMongoDBCollectionThroughput against a shared-throughput collection in PowerShell and the equivalent bash CLI command, then trace where the generic 404 self-help error is produced. Done means the error clearly identifies the shared-throughput case and provides more useful guidance in both command-line experiences.

Written by the indexing model from the issue text.