doc: begin to split apart the manual's Decl Summary section.

doc: begin to split apart the manual's Decl Summary section.

Discussed in thread starting at
<http://lists.gnu.org/archive/html/bison-patches/2009-05/msg00013.html>.
* doc/bison.texinfo (Decl Summary): Extract most of the %code
entry into...
(%code Summary): ... this new subsection, and update all
cross-references.  For readability of the patches, rewriting of
the text so it makes sense as a separate subsection will come in a
later patch.