Similar to the recently removed %before-header, add %code-top as the

Similar to the recently removed %before-header, add %code-top as the
alternative to the pre-prologue.  Mentioned at
<http://lists.gnu.org/archive/html/bison-patches/2006-10/msg00063.html>.
Also, let the prologue alternatives appear in the grammar section.
* src/parse-gram.y (PERCENT_CODE_TOP): New token.
(prologue_declaration): Move the existing prologue alternatives to...
(grammar_declaration): ... here and add %code-top.
* src/scan-gram.l (PERCENT_CODE_TOP): New token.

Clean up and extend documentation for the prologue alternatives.
* NEWS (2.3a+): Describe prologue alternatives.
* doc/bison.texinfo (Prologue): Move discussion of prologue
alternatives to...
(Prologue Alternatives): ... this new section, and extend it to discuss
all 4 directives in detail.
(Bison Symbols): Clean up discussion of prologue alternatives and add
%code-top.