define metadata in terms of unsized tail to reduce redundancy

src/dynamically-sized-types.md

@@ -8,10 +8,10 @@ r[dynamic-sized.restriction]
 Such types can only be used in certain cases:
 
 r[dynamic-sized.pointer-types]
-* [Pointer types] to <abbr title="dynamically sized types">DSTs</abbr> are sized but have twice the size of pointers to sized types, since they also store *metadata*:
-    * Pointers to slices store the number of elements; pointers to `str` store the length in bytes.
-    * Pointers to trait objects store a pointer to a vtable.
-    * Pointers to a struct or tuple with an [unsized tail] store the same metadata as a pointer to that tail.
+* [Pointer types] to <abbr title="dynamically sized types">DSTs</abbr> are sized but have twice the size of pointers to sized types, since they also store *metadata*. The type of the metadata depends on the type of the [unsized tail]:
+    * If the unsized tail is a slice, metadata stores the number of elements.
+    * If the unsized tail is a `str`, metadata stores the length in bytes.
+    * if the unsized tail is a trait object, metadata stores a pointer to a vtable.
 
 r[dynamic-sized.question-sized]
 * <abbr title="dynamically sized types">DSTs</abbr> can be provided as type arguments to generic type parameters having the special `?Sized` bound. They can also be used for associated type definitions when the corresponding associated type declaration has a `?Sized` bound. By default, any type parameter or associated type has a `Sized` bound, unless it is relaxed using `?Sized`.
@@ -27,7 +27,10 @@ r[dynamic-sized.struct-field]
 > [Variables], function parameters, [const] items, and [static] items must be `Sized`.
 
 r[dynamic-sized.tail]
-The *unsized tail* of a type is the dynamically sized component that the [metadata] of a pointer to the type describes. A [slice] (`[T]`) and a [`str`] are each their own unsized tail, described by a length; a [trait object] (`dyn Trait`) is its own unsized tail, described by a pointer to a vtable. When a struct (per [dynamic-sized.struct-field]) or a tuple has an unsized last field, its unsized tail is the unsized tail of that field. A sized type has no unsized tail.
+The *unsized tail* of a type is the dynamically sized component at the "end" of the type, after recursively descending through compound types:
+  - A [slice] (`[T]`), [`str`], and [trait object] (`dyn Trait`) are each their own unsized tail.
+  - When a struct (per [dynamic-sized.struct-field]) or a tuple has an unsized last field, its unsized tail is the unsized tail of that field.
+  - A sized type has no unsized tail.
 
 [metadata]: dynamic-sized.pointer-types
 [sized]: special-types-and-traits.md#sized