Revise documentation for div@type

[cmil]

As @lb42 notes in #70:

There is a comment "In general it is desired that encoders list and define the additional values used in the attribute type on <div> at some other place. [Instructions: TBD]" which seems rather mysterious. Surely, documentation for such values should be provided by a valList in the ODD ?

I would suggest to clarify that sentence with clear recommendations or remove it entirely.

I would also suggest to move the documentation of corpora specific values for div@type elsewhere, maybe to a wiki page in the respective corpus repo. We could link to such a page as an example. But I don't think it makes sense here to maintain a list of all custom values corpus editors may come up with.