Document "<unchanged>" and "<redacted>" values for DataSourceCredentials.connectionString

The `DataSource` model requires `DataSourceCredentials`, but the latter model is not returned from data source APIs. Since it's required, we cannot send back null (at least, the .NET generator does not support this since it's not `"x-nullable": true` and is, again, authored as required) and if we send an empty value (`connectionString` is not required), it would either err on the server or, worse, wipe out the existing credentials stored for the data source.

Instead, I'm told, the `connectionString` should be set to either "\" or "\" to avoid wiping out the credentials. These should be documented in swagger so that callers know what to pass to update a data source.

Locate the DataSource and DataSourceCredentials definitions in the repository's Azure REST API specifications and read how connectionString is currently described. Document the "" and "" values in the relevant OpenAPI definition, then verify that the generated specification clearly tells callers when to use each value.

Written by the indexing model from the issue text.