Rustdoc: the `Sized` bound on traits isn't shown

[InnocentusLime]

rustdoc doesn't show that some traits require the type to be sized, which can get a bit misleading.

For example, the docs for std::default::Default (https://doc.rust-lang.org/std/default/trait.Default.html) show

pub trait Default {
    pub fn default() -> Self;
}

While in reality it is actually

pub trait Default: Sized {
    /* Docs and stabilization attributes */
    fn default() -> Self;
}

When looking at a trait definition, rust assumes that the type the trait will be implemented for is ?Sized, so dropping Sized is incorrect.

[jyn514]

Duplicate of #24183.