Require primitives to specify descriptions in class docstring

We should probably require primitives to specify descriptions in class docstrings. Currently, all docstrings gets inherited from the parent class (and arguments merged) which I think it is OK for methods, but for the main class it is in fact pretty bad if there is no text to even tell what the primitive is doing. I think we should require it to be set as part of the primitive.

On the other hand, description is made required in metadata, so I am not sure if we should require it at the code level. Should we require it also at the metadata level then?