More changes to drop the requirement for document-uri() uniqueness

[ndw]

Issue #898 was about dropping this constraint that document-uri()s had to be unique and PR #905 was adopted to resolve it. However, I see that the the XPath specification still contains the following note:

Note:

This means that given a document node $N, the result of fn:doc(fn:document-uri($N)) is $N will always be true, unless fn:document-uri($N) is an empty sequence.

I don't believe that applies any longer, so it should be removed.

It's possible that we need to finesse the description of available documents as well. The current description was clearly written from the perspective that document URIs would be unique and there'd be a 1:1 mapping from URIs to documents.