update documentation structure so that doxygen generates nice output

[Linas Vepstas]

Log Message:
-----------
update documentation structure so that doxygen generates nice output

Modified Files:
--------------
    gnucash/src/engine:
        gnc-date.h
        gnc-engine-util.h
        gnc-numeric.h
        gnc-trace.h
        guid.h
        kvp-util-p.h
        kvp-util.h
        kvp_frame.h
        qofbackend-p.h
        qofbackend.h
        qofbook-p.h
        qofbook.h
        qofclass-p.h
        qofclass.h
        qofgobj.h
        qofid-p.h
        qofid.h
        qofinstance-p.h
        qofinstance.h
        qofobject-p.h
        qofobject.h
        qofquery-deserial.h
        qofquery-serialize.h
        qofquery.h
        qofquerycore.h
        qofsession.h
        qofsql.h