rustdoc should show the `Sized?` keyword

[japaric]

Otherwise the documentation is incorrectly stating that the trait, type parameters, etc are always sized.

Example: Deref

The trait definition should look like this:

pub trait Deref<Sized? Result> for Sized? { /* .. */ }

And the implementors section should look like this:

impl<'a, Sized? T> Deref<T> for &'a T
impl<'a, Sized? T> Deref<T> for &'a mut T
// ...

[sfackler]

Dup of #18515