Document when associated types may be omitted from dyn Trait type

[traviscross]

We accept this,

pub trait Tr {
    type Ty;
}
fn f(_: &dyn Tr<Ty = ()>) {}

and we accept this (as of rust-lang/rust#112319),

pub trait Tr {
    type Ty where Self: Sized;
}
fn f(_: &dyn Tr) {}

but we of course don't accept this:

pub trait Tr {
    type Ty;
}
fn f(_: &dyn Tr) {} //~ ERROR

The Reference should speak to this in our chapter on trait object types, but is silent on this matter.

cc @ehuss #1932 rust-lang/rust#127251