Add some comments.

Also use `Default::default()` in one `TypedArena::default()`, for
consistency with `DroplessArena::default()`.

compiler/rustc_middle/src/arena.rs

@@ -1,4 +1,4 @@
-/// This declares a list of types which can be allocated by `Arena`.
+/// This higher-order macro declares a list of types which can be allocated by `Arena`.
 ///
 /// Specifying the `decode` modifier will add decode impls for `&T` and `&[T]` where `T` is the type
 /// listed. These impls will appear in the implement_ty_decoder! macro.