Clarifying p:document, p:load wrt p:http-request

Regarding issue 145, https://github.com/xproc/specification/issues/145
I've created a draft that attempts to clarify p:document and p:load.

1. I've renamed content-type on p:load to override-content-type and
   changed its semantics accordingly.

2. I've defined the semantics of p:document in terms of p:load.

3. I've implied a connection between the semantics of p:load and
   p:http-request.

In the course of these changes, I've changed p:load (and consequently
p:document) so that they can return multiple documents in the case
where the requested resource is multipart.

See: https://ndw.github.io/specification/langspec/clarify-load/head/xproc20/diff.html#p.document
and: https://ndw.github.io/specification/langspec/clarify-load/head/xproc20-steps/diff.html#c.load

                                        Be seeing you,
                                          norm

-- 
Norman Walsh
Lead Engineer
MarkLogic Corporation
Phone: +1 512 761 6676
www.marklogic.com