doc: clean up new subsections in manual.

doc: clean up new subsections in manual.

* doc/bison.texinfo (%define Summary): Reword so it reads well as
a separate section.  For example, add an intro, and move most of
the text outside of the @deffn so it is not indented so far.
(%code Summary): Likewise.
(Table of Symbols): Reword %code entry to match the %code entry in
Decl Summary.