Improve README with more information about `urls`

README.md

@@ -131,8 +131,8 @@ Parameters with dots in their names are single strings used to organize subordin
 Parameter Name | Description
 --- | ---
 url | The url pointing to API definition (normally `swagger.json` or `swagger.yaml`). Will be ignored if `urls` or `spec` is used.
-urls | An array of API definition objects (`{url: "<url>", name: "<name>"}`) used by Topbar plugin. When used and Topbar plugin is enabled, the `url` parameter will not be parsed.
-urls.primaryName | When using `urls`, you can use this subparameter select an initial spec instead of defaulting to the first one. Useful as a query parameter to link to a specific spec.
+urls | An array of API definition objects (`{url: "<url>", name: "<name>"}`) used by Topbar plugin. When used and Topbar plugin is enabled, the `url` parameter will not be parsed. Names and URLs must be unique among all items in this array, since they're used as identifiers.
+urls.primaryName | When using `urls`, you can use this subparameter. If the value matches the name of a spec provided in `urls`, that spec will be displayed when Swagger-UI loads, instead of defaulting to the first spec in `urls`.
 spec | A JSON object describing the OpenAPI Specification. When used, the `url` parameter will not be parsed. This is useful for testing manually-generated specifications without hosting them.
 validatorUrl | By default, Swagger-UI attempts to validate specs against swagger.io's online validator. You can use this parameter to set a different validator URL, for example for locally deployed validators ([Validator Badge](https://github.com/swagger-api/validator-badge)). Setting it to `null` will disable validation.
 dom_id | The id of a dom element inside which SwaggerUi will put the user interface for swagger.