c++: improve the generated documentation

* data/lalr1.cc, data/location.cc: Improve documenting comments.
* tests/c++.at (Doxygen Documentation): Fix AT_BISON_OPTION_PUSHDEFS,
so that the generated yyerror is correct.
* tests/c++.at, tests/headers.at: Prefer %empty.

tests/headers.at

@@ -41,7 +41,7 @@ AT_DATA_GRAMMAR([$1.y],
 ]AT_YYLEX_DECLARE_EXTERN[
 %}
 %%
-dummy: /* empty */;
+dummy: %empty;
 %%
 #include <$1.h>
 ])
@@ -81,7 +81,7 @@ AT_DATA_GRAMMAR([input.y],
 ]AT_YYLEX_DEFINE[
 %}
 %%
-exp: /* empty */;
+exp: %empty;
 ]])
 
 AT_BISON_CHECK([--defines -o input.c input.y])