We refer to the unsized tail of a type, but we hadn't defined it.
Let's do that and link to the definition.

The validity rule for the metadata of a wide reference, `Box<T>`,
or raw pointer mentions `dyn Trait` and slice but had omitted
`str`.  Let's fix that.

We document that, for references and `Box<T>`, pointed-to values
with slice or `str` metadata must be no larger than `isize::MAX`.  We
hadn't required this for pointed-to values with `dyn` metadata.  It's
tempting to think this isn't necessary since we separately require
that the metadata point to a vtable generated by the compiler, which
ensures the encoded size of the erased type is OK.

But the bound is on the total size of the pointed-to value, including
any sized prefix of a type with an unsized tail.  Since the prefix
combined with the size in the vtable can push us past the limit, we
need the separate restriction.

Let's apply the rule to both cases and add an admonition to remind
ourselves of why this is needed.

We say that pointers to DSTs store metadata and what that metadata
is for pointers to slices, `str`, and trait objects.  But a struct or
tuple with an unsized tail is itself a DST, and we hadn't said what
the metadata is for pointers to these unsized types.

Now that we've defined *metadata* and *unsized tail*, let's complete
this enumeration.

We guarantee various things about the layout of pointers and
references, but we'd made no guarantees of equivalence between two
pointers to distinct unsized types that differ in the unsized tail.
Let's make some useful guarantees about this.