DS-1798: Reenabling the XSLTDissemination crosswalk.

We should re-enable the XSLTDissemination crosswalk as it is used. The DSpace default configuration is correct and does not produces the error messages that leeds to DS-1798. I added warnings in dspace.cfg and the DSpace documentation to prevent users to run in those error messages. I think this PR fixes DS-1798.