Add trait_upcasting related languages changes

Author: crlf0710

src/type-coercions.md

@@ -191,7 +191,7 @@ r[coerce.unsize]
 
 r[coerce.unsize.intro]
 The following coercions are called `unsized coercions`, since they
-relate to converting sized types to unsized types, and are permitted in a few
+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.
 
@@ -207,6 +207,9 @@ r[coerce.unsize.slice]
 r[coerce.unsize.trait-object]
 * `T` to `dyn U`, when `T` implements `U + Sized`, and `U` is [dyn compatible].
 
+r[coerce.unsize.trait-upcast]
+* `dyn T` to `dyn U`, when `T` has `U` as one of its ancestor trait.
+
 r[coerce.unsized.composite]
 * `Foo<..., T, ...>` to `Foo<..., U, ...>`, when:
     * `Foo` is a struct.