Add comment about split factors dividing extent in tutorial.
Add comment about split factors dividing extent in tutorial.


Former-commit-id: d7ddb624eb0bc96bb75b704ee947d57dfed58dd9
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide