Recommend newtypes over type aliases (#2476)

I don't know how controversial this is, but I would recommend against
using type aliases where a newtype can be used instead.

Personally, I don't like type aliases much since I feel they cause extra
indirection: I will inevitably have to go look up the alias the first
many times I encounter it. Nothing will tell me to apply the type alias
consistently, and I dislike having multiple names for the same thing.

But I'm sure some people love them, so now I'm just gently reminding
people that newtypes can be a better alternative.

src/user-defined-types/aliases.md

@@ -23,6 +23,10 @@ type PlayerInventory = RwLock<Vec<Arc<RefCell<Item>>>>;
 
 <details>
 
-C programmers will recognize this as similar to a `typedef`.
+- A [newtype](tuple-structs.html) is often a better alternative since it creates
+  a distinct type. Prefer `struct InventoryCount(usize)` to
+  `type InventoryCount = usize`.
+
+- C programmers will recognize this as similar to a `typedef`.
 
 </details>