Section on coercions fails to mention requirement on trait object creation

Rust converts a Pointer<T> to a Pointer<Trait> where T implements Trait and Pointer is a type constructor like & or Box, only if Pointer<T> is a thin pointer. The Nomicon suggests otherwise.

According to the "Coercions" section of the Nomicon:

• Pointer<T>: CoerceUnsized<Pointer<U>> for all pointer types, where T: Unsize<U>.
• Unsize<U> is automatically implemented for T if T: U.

Suppose I have two traits T1 and T2, and impl T2 for T1. Since T1: Unsize<T2>, the Nomicon claims that Pointer<T1>: CoerceUnsized<Pointer<T2>>. But I can't convert a Box<T1> to a Box<T2>; that would be a doubly-fat pointer, since it needs both the vtable for impl T2 for T1 and whatever vtable Box<T1> was using.

There was a longish thread about this; I think this post is where I finally got it right: https://users.rust-lang.org/t/trait-objects-of-trait-objects/14077/27

Start with the Nomicon's "Coercions" section and compare its statements about CoerceUnsized, Unsize, and trait-object creation with the Box to Box example in the issue. Read the linked Rust Users discussion for the settled explanation, then revise the section to state the missing thin-pointer requirement and verify that the example is accurately covered.