td/th scope attribute - rowgroup == (tbody, thead, tfoot)

http://www.w3.org/TR/xhtml2/mod-tables.html#sec_29.6 says that the
scope attribute must be one of row|col|rowgroup|colgroup, but rowgroup
is still not defined explicitly there, nor in an index.

Other portions of the document sometimes use "table section" rather
than rowgroup.

Suggestion:

In the structural model, after the list of elements, it defines some
content sets, such as Heading and Flow.  Could the table module do
that same for the content set rowgroup?  (Perhaps with a note that it
isn't used in the table above, but is sometimes used as an attribute
value, analogous to colgroup.)

-jJ