Expand a bit on clone() in Dining Philosophers

Fixes #25597

src/doc/trpl/dining-philosophers.md

@@ -674,9 +674,13 @@ let handles: Vec<_> = philosophers.into_iter().map(|p| {
 
 Finally, inside of our `map()`/`collect()` loop, we call `table.clone()`. The
 `clone()` method on `Arc<T>` is what bumps up the reference count, and when it
-goes out of scope, it decrements the count. You’ll notice we can introduce a
-new binding to `table` here, and it will shadow the old one. This is often used
-so that you don’t need to come up with two unique names.
+goes out of scope, it decrements the count. This is needed so that we know how
+many references to `table` exist across our threads. If we didn’t have a count,
+we wouldn’t know how to deallocate it.
+
+You’ll notice we can introduce a new binding to `table` here, and it will
+shadow the old one. This is often used so that you don’t need to come up with
+two unique names.
 
 With this, our program works! Only two philosophers can eat at any one time,
 and so you’ll get some output like this: