The MaxScale documentation for ReadConnRoute says the following:
router_options can contain a list of valid server roles. These roles are used as the valid types of servers the router will form connections to when new sessions are created.
Here is a list of all possible values for the router_options.
As you can see, it says that router_options can refer to a list, but it does not say what that list should look like. I assume that it means that it should be a CSV, such as:
If so, then the documentation should probably say so.