Future improvement: DocumentReferences and RelatedTopics element names unnecessarily in plural

[venilillkall]

In markup.xsd. Notice! These are the repeating, not container elements. There are couple of similar issues. I assume the current situation is not intentional(?). I realize changing this will break compatibility with existing standard. Included in pull request #64

(origin: http://jira.buildingsmart.org/browse/BCF-25)

[GeorgDangl]

See #64