doc: fix index.

http://lists.gnu.org/archive/html/bison-patches/2012-04/msg00006.html

* doc/bison.texinfo: Avoid using @def* variant with more
than the defined entity as main entity, as it results in
an incorrect index.  For instance, don't document
{return YYERROR;}, which results in a single index entry
"return YYERROR;", but rather as typed function whose
return type is "type", and whose argument list is ";".