Need section.class param entity

In trying to do some work with a customization, it struck me that it
would be very useful to have a section.class parameter entity, just
for the recursive Section element, like:

  <!ENTITY % local.section.class "">
  <!ENTITY % section.class "section %local.section.class;">

There's a precedent in that we've already got a refentry.class that
fits roughly in the same place in the hierarchy.

Having section.class would make it that much easier for users to add
Section-like elements in a customization layer, which will have the
added benefit of helping to dispel the notion that DocBook can't be
extended by adding new sub-Book/Article/Chapter sectioning elements.

This relates to the discussion about Topic, providing something more
suitable for online help, etc.-- and the issue of people not grokking
that they can use sub-Book/Article/Chapter elements (e.g. Section or
equivalent) as root elements in doc instances.