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.

data/lalr1.cc

@@ -272,8 +272,9 @@ m4_define([b4_shared_declarations],
     /// Print the state stack on the debug stream.
     virtual void yystack_print_ ();
 
-    // Debugging.
+    /// Debugging level.
     int yydebug_;
+    /// Debug stream.
     std::ostream* yycdebug_;
 
     /// \brief Display a symbol type, value and location.

data/location.cc

@@ -59,7 +59,7 @@ m4_ifdef([b4_location_file],
 # ------------------
 # Define the position and location classes.
 m4_define([b4_location_define],
-[[  /// Abstract a position.
+[[  /// A point in a source file.
   class position
   {
   public:]m4_ifdef([b4_location_constructors], [[
@@ -178,7 +178,7 @@ m4_define([b4_location_define],
     return ostr << pos.line << '.' << pos.column;
   }
 
-  /// Abstract a location.
+  /// Two points in a source file.
   class location
   {
   public: