mention that bound computation needs performance models to be calibrated

doc/chapters/advanced-examples.texi

@@ -430,7 +430,7 @@ needs to be called to destroy the dummy task afterwards. See
 @node Theoretical lower bound on execution time
 @section Theoretical lower bound on execution time
 
-For kernels with history-based performance models, StarPU can very easily provide a theoretical lower
+For kernels with history-based performance models (and provided that they are completely calibrated), StarPU can very easily provide a theoretical lower
 bound for the execution time of a whole set of tasks. See for
 instance @code{examples/lu/lu_example.c}: before submitting tasks,
 call @code{starpu_bound_start}, and after complete execution, call

doc/chapters/perf-feedback.texi

@@ -415,7 +415,7 @@ Print the DAG that was recorded
 @end deftypefun
 
 @deftypefun void starpu_bound_compute ({double *}@var{res}, {double *}@var{integer_res}, int @var{integer})
-Get theoretical upper bound (in ms) (needs glpk support detected by @code{configure} script)
+Get theoretical upper bound (in ms) (needs glpk support detected by @code{configure} script). It returns 0 if some performance models are not calibrated.
 @end deftypefun
 
 @deftypefun void starpu_bound_print_lp ({FILE *}@var{output})