Document Yacc prologue alternatives and default %destructor's and

Document Yacc prologue alternatives and default %destructor's and
%printer's as experimental.  Don't mention Java yet.  Discussed at
<http://lists.gnu.org/archive/html/bison-patches/2006-12/msg00002.html>.
* NEWS (2.3a+): Say they're experimental.  Remove any mention of Java.
(2.3a): Annotate this entry to say the old forms of these features were
also experimental.
* doc/bison.texinfo (Prologue Alternatives, Freeing Discarded Symbols,
Bison Symbols): Say they're experimental.  Comment out any mention
of Java (we'll want this back eventually).