I added clarification (That simplified example is just for illustrating contagious borrow issue. The *`total_score` is analogous to a complex state that exists

I added clarification (That simplified example is just for illustrating contagious borrow issue. The *`total_score` is analogous to a complex state that exists in real applications*. Same for subsequent examples. Just summing integer can use `.sum()` or local variable. Simple integer mutable state can be workarounded using `Cell`.)