Outputs/values expected to be set after operation are not shown in the templates documentation, as this one: https://learn.microsoft.com/en-us/azure/templates/microsoft.managedidentity/userassignedidentities?pivots=deployment-language-bicep.

They are shown in the response section of the REST API documentation though:

I don't know if it's fully accurate, but I took some samples and it seems correct to get a grasp of what you can expect to be set after the operation. There's no direct link between the template and API docs, so you have to look up the API docs for each operation you're interested in.