fwi: adjust input size to meet timing constraints

The previous iniput size for both granularity and strong scaling tests
where too big to meet the timing constrains needed for garlic. This
patch sets a new, smaller, input size.

Also, a minor cleanup is applied to the rest of the fwi experiments
and figures.

garlic/exp/fwi/strong_scaling_io.nix

@@ -1,3 +1,10 @@
+# Strong scaling test for FWI variants based on tasks with and without I/O.
+# This experiment solves a computationally expensive input which brings the
+# storage devices to saturation when I/O is enabled.  the same input us run
+# without I/O for comparison purposes..  Also, the experiments are runt for a
+# range of block sizes deemed as efficient according to the granularity
+# experiment.
+
 {
   stdenv
 , stdexp