Apply review comments to trait upcasting description

src/type-coercions.md

@@ -199,6 +199,10 @@ relate to converting types to unsized types, and are permitted in a few
 cases where other coercions are not, as described above. They can still happen
 anywhere else a coercion can occur.
 
+r[coerce.unsize.misnomer]
+> Note: "unsizing" is a bit of a misnomer,
+> since this covers unsized->unsized coercions too.
+
 r[coerce.unsize.trait]
 Two traits, [`Unsize`] and [`CoerceUnsized`], are used
 to assist in this process and expose it for library use. The following
@@ -213,6 +217,8 @@ r[coerce.unsize.trait-object]
 
 r[coerce.unsize.trait-upcast]
 * `dyn T` to `dyn U`, when `U` is one of `T`'s supertraits.
+* `dyn T` to `dyn U` when `U` is one of `T`'s [supertraits].
+    * This allows dropping auto traits, i.e. `dyn T + Auto` to `dyn U` is allowed.
 
 r[coerce.unsized.composite]
 * `Foo<..., T, ...>` to `Foo<..., U, ...>`, when:
@@ -330,3 +336,4 @@ precisely.
 [`Unsize`]: std::marker::Unsize
 [`CoerceUnsized`]: std::ops::CoerceUnsized
 [method-call expressions]: expressions/method-call-expr.md
+[supertraits]: items/traits.md#supertraits