make it clear that performance models are per size

doc/starpu.texi

@@ -1159,8 +1159,11 @@ same. This is very true for regular kernels on GPUs for instance (<0.1% error),
 and just a bit less true on CPUs (~=1% error). This also assumes that there are
 few different sets of data input/output sizes. StarPU will then keep record of
 the average time of previous executions on the various processing units, and use
-it as an estimation. It will also save it in @code{~/.starpu/sampling/codelets}
-for further executions.  The following is a small code example.
+it as an estimation. History is done per task size, by using a hash of the input
+and ouput sizes as an index.
+It will also save it in @code{~/.starpu/sampling/codelets}
+for further executions, and can be observed by using the
+@code{starpu_perfmodel_display} command.  The following is a small code example.
 
 @cartouche
 @smallexample