I've just found another useful tool, for those dealing with schemas and documentation...
Lars Marius Garshol has had this tool out there since 2001, but I just ran across it trying to search for Content Model documentation.
dtddoc is a DTD documentation generator which can read a DTD and associated documentation written in XML and generate nicely formatted HTML documentation or, experimentally, DocBook RefEntry documentation.
It can create HTML, DocBook, and XML Topic Map (XTM) documentation, as well as read a DTD and generate a skeleton documentation file.
It's available at: http://www.garshol.priv.no/download/software/dtddoc/. Lars also has a cool list of XML tools available at: http://www.garshol.priv.no/download/xmltools/cat_ix.html
From Lars' tools page, the following DTD documentation tools are listed:
Product Vendor Platforms dtddoc Lars Marius Garshol Python DTDDoc Stefan Champailler Java DTDParse Norman Walsh Perl LiveDTD Robert Stayton Perl perlSGML Earl Hood Perl
UPDATE: There's an informative article on this subject at: http://builder.com.com/5100-6315-1051811.html
I would think there are similar tools for documenting XSD and RNG schemas. If you know of any, please send in your comments!
See also: XML