clarify NLL error description

Author: lqd

posts/inside-rust/2023-09-25-polonius-update.md

@@ -52,7 +52,11 @@ fn main() {
 }
 ```
 
-Here, the loan created on iteration N _may be used_ in the `None` path on iteration N+1, however the borrowed variable at that iteration is not the same.
+The current borrow checker rejects this code. It does this because it sees that there is a loan of `temp` to call `temp.maybe_next()`. It also sees that this loan can flow around the loop — in particular, the loan is referenced by `v`, which is then stored into `temp`. Therefore, it reports an error when we attempt to mutably borrow `temp` at the top of the loop, since there may be a loan still existing from a prior iteration.
+
+*However*, a more careful read shows that, while the borrow may flow around the loop, it only does so on the `Some` path, and on that path `temp` is overwritten. This means that the `temp` we will be borrowing on the next iteration is in fact a different location than the one we borrowed on the previous iteration. On the `None` path, meanwhile, the loan ends.
+
+This kind of "case by case" distinguishing — seeing that the flow only occurs on one path, and that on that path there is a reassignment, requires more precision than the current borrow checker can achieve.
 
 Issues like "NLL problem case #3", issue #47680 and others, were therefore deferred from NLLs, and left as future work, [Polonius](http://smallcultfollowing.com/babysteps/blog/2018/04/27/an-alias-based-formulation-of-the-borrow-checker/).