Clarify documentation on RedisObject creation

[mbaruh]

The __set_name__ docstring of RedisObject reads:

This class MUST be created as a class attribute, otherwise it will raise
exceptions whenever a method is used. This is because it uses this
method to create a namespace like `MyCog.my_class_attribute` which is
used as a hash name when we store stuff in Redis, to prevent collisions.

This gives the impression that a RedisObject is limited to being assigned to a class variable. In reality, it is possible to assign it to an instance attribute by specifying the namespace argument at init.

The docs should be amended to reflect that.