Rollup merge of #87904 - kpreid:unsize, r=jyn514

Reword description of automatic impls of `Unsize`.

The existing documentation felt a little unhelpfully concise, so this change tries to improve it by using longer sentences, each of which specifies which kinds of types it applies to as early as possible. In particular, the third item starts with “Structs ...” instead of saying “Foo is a struct” later.

Also, the previous list items “Only the last field has a type involving `T`” and “`T` is not part of the type of any other fields” are, as far as I see, redundant with each other, so I removed the latter.

I have no particular knowledge of `Unsize`; I have attempted to leave the meaning entirely unchanged but may have missed a nuance.

Markdown preview of the edited documentation:

> All implementations of `Unsize` are provided automatically by the compiler.
> Those implementations are:
>
> - Arrays `[T; N]` implement `Unsize<[T]>`.
> - Types implementing a trait `Trait` also implement `Unsize<dyn Trait>`.
> - Structs `Foo<..., T, ...>` implement `Unsize<Foo<..., U, ...>>` if all of these conditions
>   are met:
>   - `T: Unsize<U>`.
>   - Only the last field of `Foo` has a type involving `T`.
>   - `Bar<T>: Unsize<Bar<U>>`, where `Bar<T>` stands for the actual type of that last field.

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