Add clarification

Co-authored-by: Niko Matsakis <[email protected]>

Author: traviscross

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 types to unsized types, and are permitted in a few
+relate to converting sized types to unsized types or converting from one unsized type to another, 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.