Fixes for missing or bugs in the custom token STS docs (#1479)

- Adds that `idmp-` string is added to `ROLE_ID` when generating an ARN
- Updates claims example in the plugin docs to be JSON instead of basic
comma-delimited key-value pairs

source/developers/security-token-service/AssumeRoleWithCustomToken.rst

@@ -67,6 +67,9 @@ This endpoint supports the following query parameters:
 
        See :envvar:`MINIO_IDENTITY_PLUGIN_ROLE_ID` or :mc-conf:`identity_plugin role_id <identity_plugin.role_id>` for more information.
 
+       Note that MinIO automatically prepends ``idmp-`` to a configured ``ROLE_ID`` when generating the RoleArn.
+       Include that string with the ``ROLE_ID`` if required.
+
    * - ``DurationSeconds``
      - integer
      - *Optional*