DocBook5 is the current development tree of DocBook. Thus the package needs to be updated, whenever a new version is released. Usually there is not much to do. If you are interested in this standard, then this is probably a package of interest. The user base will grow whenever people or packages move from v4 to DocBook5.
There are a few open bug reports about the upgrade stylesheet from DB4 to DB5. The upstream version however might already contain fixes.
The package description is: standard XML documentation system for software and systems
DocBook 5 is a general purpose XML schema. That is, it
contains the "DocBook" document structure. This is used by authors
or editors writing documents in the DocBook XML format. DocBook was
designed for books, articles, or reference documentation about
technical matters or software (though it is by no means limited to
these applications). It has emerged as an open standard in the
software industry, and is the documentation system of choice for
many free software projects.
DocBook 5.0 is a complete rewrite of the well-known DocBook 4 XML
schema/DTD, introducing a quite large number of backwards-incompatible
changes. But the basic idea behind DocBook is still the same, and
almost all element names are unchanged.
This package contains the XML RelaxNG schema (with some additional
Schematron assertions), W3C XML Schema (XSD) as well as the
Document Type Definition (DTD) for DocBook, which all describe the
formal structure for documents following this format.
You will need other packages in order to edit, validate (libxml2-utils)
or format (docbook-xsl-ns) DocBook 5 documents.