Document the field last of CoalesceBy

This behavior is not obvious at first glance of `Iterator::next`.
In it, after `let init =`, `last` is `Some(None)` in both cases. With `try_fold`, if `iter` is not exhausted then we have `*last = Some(Some(..))` (otherwise it remains `Some(None)`).

src/adaptors/coalesce.rs

@@ -10,6 +10,9 @@ where
     C: CountItem<I::Item>,
 {
     iter: I,
+    /// `last` is `None` while no item have been taken out of `iter` (at definition).
+    /// Then `last` will be `Some(Some(item))` until `iter` is exhausted,
+    /// in which case `last` will be `Some(None)`.
     last: Option<Option<C::CItem>>,
     f: F,
 }