A slight change to SET_LINENO-less tracing.

This makes things a touch more like 2.2.  Read the comments in
Python/ceval.c for more details.

Include/frameobject.h

@@ -29,6 +29,8 @@ typedef struct _frame {
     PyObject *f_exc_type, *f_exc_value, *f_exc_traceback;
     PyThreadState *f_tstate;
     int f_lasti;		/* Last instruction if called */
+    /* As of 2.3 f_lineno is only valid when tracing is active (i.e. when
+       f_trace is set) -- at other times use PyCode_Addr2Line instead. */
     int f_lineno;		/* Current line number */
     int f_restricted;		/* Flag set if restricted operations
 				   in this scope */