Feature/documentation tables xcore - see PR#438

[hrennau]

Tried to address all issues from the list PR#438. A couple of items were not resolved:

• Item 1. - I did not find the comma reported, thus could not remove anything
• Item 7. - I reduced the repetition of local type descriptions - no duplicates among the types belonging to the same schema, and the types are no ordered. But I did not remove duplicates across different domains, as this would require an undue (as it seems to me) effort.
• Item 8. - Concerning the formatting of cardinalities, I did not understand what is required, in particular did not understand "spaces should be removed".